Photoluminescence and Energy Transfer of Ce3+, Tb3+, and Eu3+

Jul 16, 2015 - ABSTRACT: A series of Ce3+, Tb3+ codoped and Ce3+, Tb3+, Eu3+ tridoped KBaY(BO3)2 (KBYB) phosphors are synthesized by...
3 downloads 0 Views 7MB Size
Download PDF
Article pubs.acs.org/IECR

Photoluminescence and Energy Transfer of Ce3+, Tb3+, and Eu3+ Doped KBaY(BO3)2 as Near-Ultraviolet-Excited Color-Tunable Phosphors Xinguo Zhang*,†,‡ and Menglian Gong‡ †

School of Chemistry and Chemical Engineering, Guangxi University, Nanning 530004, China State Key Laboratory of Optoelectronic Materials and Technologies, School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou 510275, China

‡

ABSTRACT: A series of Ce3+, Tb3+ codoped and Ce3+, Tb3+, Eu3+ tridoped KBaY(BO3)2 (KBYB) phosphors are synthesized by the solid-state method. The codoped and tridoped phosphors absorb near-ultraviolet (NUV) light through 4f−5d transitions of Ce3+, followed by blue emissions of Ce3+ along with sensitized Tb3+green and Eu3+red emission, respectively. The energy-transfer process is discussed in terms of both the luminescence spectra and decay curves. Because of the long Y−Y distance in KBYB structure, high Tb3+ content is required to form a Tb3+ bridge for a complete Ce3+ → Tb3+ → Eu3+ energy transfer. By controlling the Ce3+/Tb3+/Eu3+ ratio, the color of the phosphor varied from blue (0.173, 0.087) to green (0.341, 0.570) and eventually to red (0.589, 0.374). The results indicated that the codoped and tridoped KBYB exhibited broadband NUV absorption and blue−green−red tunable emission, which might serve as down-converted phosphors for NUV light-emitting diodes. sensitizer and activator.11 As discussed by Blasse12 and Setlur,13 Tb3+/Gd3+ could be used as a bridge between Ce3+ and Eu3+ to minimize the MMCT effect and maximize energy-transfer (ET) probability, which indicates that phosphors with strong narrow line red emission and broadband NUV excitation can be synthesized using a Ce3+ → Tb3+ → Eu3+ scheme. Novel orange/red phosphors, such as Sr3Ln(PO4)3:Eu2+,Tb3+, Sm3+,14 Ba2Ln(BO3)2Cl:Eu2+, Tb3+, Eu3+15 and Na2Y2B2O7:Ce3+, Tb3+, Eu3+16 have been reported. A buetschliite-type rare-earth borate KBaY(BO3)2, which crystallizes in the trigonal space group R3̅m, was first reported by Gao et al. in 2011.17 Luminescent properties of blue-to-red emitting KBaY(BO3)2:Ce3+, Mn2+18 and green-emitting KBaY(BO3)2:Tb3+19,20 phosphors were studied recently. In this work, a Ce3+ → Tb3+ → Eu3+ scheme is introduced into KBaY(BO3)2 to develop NUV-excitable green and red phosphor. The luminescent properties and energy-transfer process were investigated. The effect of Tb3+ content on luminescent properties of Ce3+, Tb3+, Eu3+ tridoped KBaY(BO3)2 were systematically studied to find out the optimal Tb3+ concentration to form a Tb3+ bridge.

1. INTRODUCTION In the last few decades, rare earth ion (RE3+)-doped inorganic luminescent materials have been widely investigated for their potential lighting, display fields, and biological applications. One of the interesting characteristics of RE3+ ions is their sharp transitions arising because of intra 4f transitions, which are shielded by the outer 5s and 5p orbitals and are barely affected by the external environment or crystal field.1,2 At present, the production of white light by combining red, green, and blue tricolor phosphors with a near-ultraviolet (NUV) diode is highly favored.3,4 Therefore, developing novel red, green, and blue phosphors with efficient NUV absorption is of great importance for further light-emitting diode (LED) development. The potential green and red LED phosphors should have a strong absorption in the NUV region and an intense emission in red. Conventional green and red phosphors, such as LaPO4:Tb3+ and Y2O2S:Eu3+, possess high quantum efficiency and high color purity, but their excitation band in the NUV region is too narrow and weak because of 4f−4f forbidden transitions.5,6 From a spectroscopic point of view, Eu2+-doped (oxy)nitride phosphors, e.g., β-SiAlON:Eu2+ and CaAlN3:Eu2+, can meet the requirements.7,8 However, critical synthetic requirements of air-sensitive metal nitrides make them unfavorable candidates for competitive products. Sensitization is a good strategy to greatly expand NUV excitation band and enhance emission intensity of RE3+ ions.9,10 Activators like Tb3+ and Eu3+, which are generally used as green- and red-emitting ions with 4f−4f forbidden transitions, can be effectively sensitized by Ce3+ and Eu2+ (sensitizers) with 4f−5d allowed transitions. However, a direct sensitization of Eu3+ by Ce3+ is not feasible because of the existence of metal− metal charge transfer (MMCT, i.e., Ce3+ + Eu3+ → Ce4+ + Eu2+), which strongly quenches the luminescence of both © 2015 American Chemical Society

2. EXPERIMENTAL SECTION All samples with a hypothetical composition of KBaY(1−x−y−z)(BO3)2:xCe3+, yTb3+, zEu3+ were synthesized using a conventional solid-state reaction method. The stoichiometric starting materials BaCO3 (99.9%), K2CO3 (99.9%), Y2O3 (99.99%), H3BO3 (analytical reagent), CeO2 Received: Revised: Accepted: Published: 7632

April 28, 2015 June 29, 2015 July 16, 2015 July 16, 2015 DOI: 10.1021/acs.iecr.5b01576 Ind. Eng. Chem. Res. 2015, 54, 7632−7639

Article

Industrial & Engineering Chemistry Research

Figure 1. (a) XRD patterns of KBaY(1−x−y−z)(BO3)2:xCe3+, yTb3+, zEu3+ samples; (b) crystal structure of KBaY(BO3)2; and (c) the coordination environment of YO6 and (Ba/K)O9 polyhedrons.

Figure 2. PL and PLE spectra of representative Ce3+-doped (a); Tb3+-doped and Ce3+, Tb3+ codoped (b); and Eu3+-doped and Ce3+, Tb3+, Eu3+ tridoped KBYB phosphor (c).

920 time-resolved and steady state fluorescence spectrometers (Edinburgh Instruments) equipped with a 450 W Xe lamp, TM300 excitation monochromator, and double TM300 emission monochromators and Red PMT. The time-resolved luminescence data were measured by an Edinburgh FLS980 photoluminescence spectrometer with a nanosecond−microsecond flashlamp and R928P detector. The spectral resolution is about 0.5 nm in UV−visible.

(99.99%), Tb4O7 (99.99%), and Eu2O3 (99.99%) were mixed and ground thoroughly in an agate mortar. A 5 mol % excess amount of boric acid was used to compensate an evaporation loss during heating. After being thoroughly ground, the mixed raw materials were preheated at 500 °C for 5 h in air, then reground and calcined in a tube furnace at 920 °C for 8 h in a CO reducing atmosphere. Final products were obtained after furnace-cooling to room temperature and again grinding into powders. X-ray powder diffraction (XRD) patterns of the products were recorded on a Rigaku D/max-IIIA diffractometer with Cu Kα radiation (λ = 1.5403 Å). The room-temperature and temperature-dependent photoluminescence excitation (PLE) and photoluminescence (PL) spectra were measured by FLS-

3. RESULTS AND DISCUSSION 3.1. XRD Patterns and Crystal Structure. Figure 1a shows XRD patterns of KBaY(1−x−y−z)(BO3)2:xCe3+, yTb3+, zEu3+ samples. All the observed diffraction peaks are well indexed to the phases of KBaY(BO3)2, and no second phase is 7633

DOI: 10.1021/acs.iecr.5b01576 Ind. Eng. Chem. Res. 2015, 54, 7632−7639

Article

Industrial & Engineering Chemistry Research

17.781(8) Å, V = 457.8(3) Å3, Z = 3. The ionic radii for the sixcoordinated Y3+, Ce3+, Tb3+, and Eu3+ are 0.90, 1.01, 0.92, and 0.95 Å, respectively.21 Thus, because of the similar ionic radius and valence, the RE ion dopants (Ce3+, Tb3+, Eu3+) were expected to replace Y3+ sites. As shown in Figure 1c, the Ba and K atoms share the same site in the proportion of 0.492:0.508, which is 9-coordinated with Ba/K−O bond lengths of 2.83−2.98 Å. The B atom is triangularly coordinated to O atoms to form BO3 triangles. The Y atom is six-coordinated by O atoms to form a slightly distorted octahedron. The O−Y−O angle deviates from the orthogonality by about 4°, and the six Y−O bond distances are equal to 2.249 Å. The nearest Y−Y distance is 5.452(6) Å; this value is close to that of K2Y(WO4)(PO4) (5.501(7) Å), which is reported to have long Y−Y distance. It is known that Tb3+ → Eu3+ energy transfer occurred by a (super)exchange mechanism, which has a sharper distance versus MMCT quenching process.22 The critical distance (Rc) for Eu3+ MMCT quenching of Ce3+ luminescence is reported as 12−14 Å, and that for Tb3+ → Eu3+ energy transfer is about 5−7 Å.23,24 Because of the long distance between Y sites, a high Tb3+ concentration is expected in order to form a Tb3+ bridge and maximize Ce3+ → Tb3+ → Eu3+ ET versus MMCT. 3.2. Photoluminescence Properties. The relationship of site occupancy and luminescent property in Ce3+-doped KBaY(BO3)2 has been systematically investigated by Yan and co-workers.18 Figure 2a shows the PLE and PL spectra of Ce3+monodoped KBYB (KBYB:0.01Ce3+). The PLE spectrum is a strong and broad band ranging from 300 to 420 nm and peaking at 364 nm, which is attributed to Ce3+ 4f → 5d transition and matched well with the emission of NUV chips. The PL spectrum shows a broad asymmetric emission band peaked at 419 nm with a shoulder at 394 nm. The optimal Ce3+ concentration is 0.01 for the KBYB:xCe3+ sample by referring to the work of Yan and co-workers,18 which is chosen as Ce3+doped content (x) for further Ce3+, Tb3+ codoped and Ce3+, Tb3+, Eu3+ tridoped KBYB samples. PL and PLE spectra of Tb 3+ -monodoped KBYB (KBYB:0.10Tb 3 + ) and Ce 3 + , Tb 3 + codoped KBYB (KBYB:0.01Ce3+, 0.05Tb3+) are shown in Figure 2b. The PL spectrum of KBYB:0.10Tb3+ consists of typical sharp peaks of the 4f−4f transitions of Tb3+ at 489 and 497 nm (5D4 → 7F6), 544 and 553 nm (5D4 → 7F5), ∼586 nm (5D4 → 7F4), and ∼625 nm (5D4 → 7F3). The PLE spectrum of KBYB:0.10Tb3+

Figure 3. PL spectra of KBYB:0.01Ce3+, yTb3+ phosphors under 364 nm excitation. Inset is the dependence of Tb3+ emission intensity and Ce3+ → Tb3+ ηET on Tb3+ concentration (y).

observed, indicating that the doping ions do not cause significant changes in the host. Moreover, the KBYB:xCe3+, yTb3+, zEu3+ phases show increasing intensity ratio of (110): (015) planes with rising Tb3+ content (y) compared to the reference reported by Gao et al.,17 which means that the phosphor particles might prefer to orient along the (110) axis when Y3+ is generally replaced by Tb3+. It is found that the (110):(015) ratio for y = 0.9 is lower than that of y = 0.7. As reported by Yan and co-workers,20 the intensity of the (110) peak is higher than that of the (015) peak in the Tb3+ fully substituted KBaTb(BO3)2 XRD pattern, which is iso-structural with KBaY(BO3)2. Therefore, the phenomenon that (110) peak shows comparable intensity with (015) peak for y = 0.9 might be due to the tendency of forming iso-structural KBaTb(BO3)2 when Tb3+ doped content (y) is very high (y = 0.9−1.0) in KBaY(1−x−y−z)(BO3)2:xCe3+, yTb3+, zEu3+ (x = 0.01, z = 0.01) Figure 1b presents the structure diagram of KBaY(BO3)2. The structure of KBaY(BO3)2 is built by two-dimensional layers: within the ab plane, the BO3 triangles and the YO6 octahedra are linked together by sharing the common oxygen vertices to generate a two-dimensional (YB2O6)∞ double layer. These layers, stacking along the c direction with model ABCABC..., are bridged by the Ba/K ions to form a threedimensional framework.17 KBaY(BO3)2 crystallizes in the trigonal space group R3̅m with a = 5.4526(12) Å, c =

Figure 4. Dependence of (Is0/Is) on Ca/3 of Ce3+ on Ca/3 (a = 6, 8, and 10) in KBYB:0.01Ce3+, yTb3+. 7634

DOI: 10.1021/acs.iecr.5b01576 Ind. Eng. Chem. Res. 2015, 54, 7632−7639

Article

Industrial & Engineering Chemistry Research

Figure 5. PL spectra of KBYB:0.01Ce3+, yTb3+, 0.01Eu3+ phosphors under 364 nm excitation. Inset is the variation of Tb3+ and Eu3+ emission intensity on Tb3+ concentration (y).

Figure 6. Decay curves for the luminescence of Ce3+ in KBYB:0.01Ce3+, yTb3+ (a) and Tb3+ in KBYB:0.01Ce3+, yTb3+, 0.01Eu3+ samples (b).

Figure 2c demonstrates the PLE and PL spectra of Eu3+ single-doped KBYB (KBYB:0.10Eu3+). The PLE spectrum of KBYB:0.10Eu3+ detected at 592 nm consists of a broad band which is attributed to the O2−−Eu3+ charge−transfer band (CTB) and a group of sharp lines in the longer wavelength region. The sharp excitation peaks between 300 and 450 nm are assigned to the 4f−4f forbidden transitions of Eu3+ in the host lattice. The strongest excitation line at 394 nm contributes to the 7F0−5L6 transition. Under the excitation of 394 nm, the phosphor KBYB:0.10Eu3+ exhibits an orange-red emission. The spectrum consists of several emission lines in the region from 570 to 700 nm, originating from the transitions of the excited state 5D0 to the ground states 7FJ (J = 0−4). It is well-known

consists of three broad bands at 250, 270, and 289 nm corresponding to 4f−5d transitions of Tb3+. Sharp and small peaks corresponding to 4f−4f transitions of Tb3+ are also observed at the wavelength region of 300−500 nm. Under 364 nm excitation, the Ce3+, Tb3+ codoped KBYB (KBYB:0.01Ce3+, 0.05Tb3+) sample exhibits both the weakened broad 5d−4f blue emissions of Ce3+ and the enhanced, sharp 4f−4f green emissions of Tb3+. Meanwhile, the 4f−5d excitation bands of Ce3+ are observed in the PLE spectrum monitored at the Tb3+ emission (λem = 544 nm) in addition to the 4f−5d excitation bands of Tb3+. These spectra are evidence of energy transfer from Ce3+ to Tb3+ in the KBYB:Ce3+, Tb3+ system. 7635

DOI: 10.1021/acs.iecr.5b01576 Ind. Eng. Chem. Res. 2015, 54, 7632−7639

Article

Industrial & Engineering Chemistry Research

The PL and PLE spectra of Ce3+, Tb3+, Eu3+ tridoped KBYB (KBYB:0.01Ce3+, 0.50Tb3+, 0.01Eu3+) are also depicted in Figure 2c. Upon Ce3+ excitation at 364 nm, the phosphor exhibits intense characteristic emission lines of Tb3+ and Eu3+ ions, while the Ce3+ emission band is almost vanished, indicating that efficient Ce3+−Tb3+−Eu3+ energy transfer is realized. The PLE spectrum was recorded while monitoring the 592 nm red emission of Eu3+ ions, which is similar to that of KBYB:Ce3+, Tb3+, reflecting the successful sensitization of Eu3+ emission by Ce3+ through Tb3+ in the KBYB:Ce3+, Tb3+, Eu3+ system. A series of Ce3+, Tb3+ codoped KBaY(BO3)2 samples were synthesized to further investigate the energy-transfer process between Ce3+ and Tb3+. Figure 3 shows the PL spectra of KBYB:0.01Ce3+, yTb3+ (y = 0, 0.01, 0.03, 0.05, 0.10, 0.20, 0.30, 0.40, and 0.50) phosphors, and the inset demonstrates the dependence of Tb3+ emission intensity on Tb3+ concentration (y). Under 364 nm UV light, both blue broadband emission of Ce3+ and green sharp emission peaks of Tb3+ are observed. The Ce3+ emission intensity generally decreases with increasing Tb3+ content, which is due to the enhancement of Ce3+ → Tb3+ energy transfer. The Tb3+ emission intensity increases initially and begins to decrease at y > 0.2, which could be attributed to Tb3+−Tb3+ internal concentration quenching. The variation of Ce3+ and Tb3+ emission indicates that tunable color from blue to green can be realized by appropriately adjusting the Ce3+/ Tb3+ ratio. The mechanism of Ce3+ → Tb3+ energy transfer could be determined by the relationships of (Is0/Is) f Ca/3, which are illustrated in Figure 4. By consulting the fitting factor R, the relation (Is0/Is) f C6/3 has the best fitting, implying that the dipole−dipole interaction is applied for Ce3+ → Tb3+ energy transfer in KBaY(BO3)2. The energy-transfer efficiency (ηET) of Ce3+ → Tb3+ can be calculated using the following equation:26

Table 1. Calculated CIE Coordinates of KBaY(BO3)2:xCe3+, yTb3+, zEu3+ Phosphors serial number 1 2 3 4 5

6 7 8 9 10 11 12 13 14 15

sample composition KBYB:0.01Ce3+ KBYB:0.01Ce3+, KBYB:0.01Ce3+, KBYB:0.01Ce3+, KBYB:0.01Ce3+, KBYB:0.01Ce3+, KBYB:0.01Ce3+, KBYB:0.01Ce3+, KBYB:0.01Ce3+, KBYB:0.01Ce3+, KBYB:0.01Ce3+, KBYB:0.01Ce3+, KBYB:0.01Ce3+, KBYB:0.01Ce3+, KBYB:0.01Ce3+, KBYB:0.01Ce3+, KBYB:0.01Ce3+, KBYB:0.01Ce3+, KBYB:0.01Ce3+,

0.01Tb3+ 0.03Tb3+ 0.05Tb3+ 0.10Tb3+ 0.20Tb3+ 0.30Tb3+ 0.40Tb3+ 0.50Tb3+ 0.05Tb3+, 0.10Tb3+, 0.20Tb3+, 0.30Tb3+, 0.40Tb3+, 0.50Tb3+, 0.60Tb3+, 0.70Tb3+, 0.80Tb3+, 0.90Tb3+,

0.01Eu3+ 0.01Eu3+ 0.01Eu3+ 0.01Eu3+ 0.01Eu3+ 0.01Eu3+ 0.01Eu3+ 0.01Eu3+ 0.01Eu3+ 0.01Eu3+

CIE (x)

CIE (y)

0.173 0.233 0.276 0.325 0.336 0.337 0.341 0.341 0.340 0.288 0.343 0.379 0.419 0.473 0.489 0.502 0.538 0.559 0.589

0.087 0.260 0.398 0.506 0.515 0.547 0.562 0.570 0.568 0.410 0.496 0.502 0.484 0.441 0.433 0.436 0.406 0.392 0.374

ηET = 1 −

I I0

where I0 and I are the luminescence intensity of the Ce3+ in the absence and presence of Tb3+, respectively. The Ce3+ → Tb3+ energy-transfer efficiencies in KBYB:0.01Ce3+, yTb3+ are calculated and shown in Figure 3 inset. With increasing Tb3+ content, the value of ηET increased remarkably. The energytransfer efficiency is about 85% when y = 0.3, and the energy transfer is close to saturation. The results confirm that Ce3+ → Tb3+ energy transfer occurs in KBaY(BO3)2 host. As suggested by Setlur,13 both Ce3+ and Eu3+ doped contents in Ce3+ → Tb3+ → Eu3+ energy transfer should remain low to minimize the probability of MMCT process. In our previous studies of GdBO3:Ce3+, Tb3+, Eu3+27 and Y2SiO5:Ce3+, Tb3+, Eu3+,28 it was found that the optimal Eu3+ content is about 1%; further increase of Eu3+ content will lead to significant concentration quenching. Thus, the Eu3+ concentration (z) for KBaY(BO3)2:xCe3+, yTb3+, zEu3+ is set as 0.01. It is important to analyze the required Tb3+ concentration for forming a Tb3+ bridge for a complete Ce3+ → Tb3+ → Eu3+ energy transfer. In order to investigate the effect of Tb3+ concentration on Ce3+ → Tb3+ → Eu3+ energy transfer, PL spectra of KBYB:0.01Ce3+, yTb3+, 0.01Eu3+ (y = 0.05, 0.10, 0.20, 0.30, 0.40, 0.50) samples under 364 nm excitation are shown in Figure 5. When Tb3+ content is relative low (y = 0.05, 0.10), the corresponding Ce3+ emission intensity decreases remarkably while Tb3+ emission intensity greatly enhances and

Figure 7. CIE diagrams of KBaY(BO 3 ) 2 :xCe 3+, yTb 3+ , zEu 3+ phosphors. Insets are digital photographs of samples under a 365 nm UV lamp.

that 5D0 → 7F2 electronic dipole transition (∼614 nm) is super sensitive to the symmetry of occupied site, while 5D0 → 7F1 magnetic dipole transition (∼592 nm) is insensitive to that. In a site with an inversion symmetry, the magnetic dipole transition 5 D0 → 7F1 is dominant, while in a site without inversion symmetry, the 5D0 → 7F2 electronic transition becomes the strongest one.25 Therefore, the (5D0 → 7F2)/(5D0 → 7F1) intensity ratio can be used as a measure of the site symmetry of Eu3+. The asymmetry ratio of KBYB:0.10Eu3+ is 0.751, indicating that Eu3+ is occupying an inversion symmetry site (YO6), which is consistent with the above discussion of KBaY(BO3)2 structure. 7636

DOI: 10.1021/acs.iecr.5b01576 Ind. Eng. Chem. Res. 2015, 54, 7632−7639

Article

Industrial & Engineering Chemistry Research

Figure 8. Schematic diagram of Ce3+ → Tb3+ → Eu3+ energy transfer in KBaY(BO3)2.

Figure 9. PL spectra of KBYB:0.01Ce3+, 0.10Tb3+ green phosphor with various temperatures under 365 nm excitation. Inset shows the normalized PL intensity as a function of temperature.

Figure 10. PL spectra of KBYB:0.01Ce3+, 0.80Tb3+, 0.01Eu3+ red phosphor with various temperatures under 365 nm excitation. Inset shows the normalized PL intensity as a function of temperature.

Eu3+ emission almost vanishes. The PL spectra demonstrates that Ce3+ → Tb3+ → Eu3+ energy transfer could not occur when Tb3+ content is too low to make it act as a bridge between Ce3+ and Eu3+. In this case, the Ce3+−Tb3+ ET process and Ce3+− Eu3+ MMCT process dominate. The Tb3+ green emission intensity reaches its maximum at y = 0.20, whereas Eu3+ red emission merges. Tb3+ emission decreases with rising Tb3+ content, while Eu3+ emission intensity reaches its maximum at y = 0.40 and then slowly decreases. The Tb3+ emission is not obvious, and the shape of the PL spectra remains unchanged when y ≥ 80%, indicating that the successful formation of a Tb3+ bridge in KBaY(BO3)2. The required Tb3+ concentration (≥80%) is close to the value reported in Ba2(Y, Gd, Lu)(BO3)2Cl:Eu2+, Tb3+, Eu3+ (∼90%), which agrees with the prediction in section 3.1. 3.3. Energy Transfer and CIE Coordinates. To further validate the Ce3+ → Tb3+ and Tb3+ → Eu3+ energy transfer in KBYB, Ce3+ and Tb3+ decay lifetimes in Ce3+, Tb3+ codoped and Ce3+, Tb3+, Eu3+ tridoped KBYB phosphors were

measured, respectively. Energy transfer from Ce3+ to Tb3+ and from Tb3+ to Eu3+ is attributed to multipolar interaction between donor and acceptor ions. As discussed by Dexter, the energy-transfer probability via multipolar interaction can be described by29 P(R ) ∝

QA b

R τD

∫

fD (E)FA(A) Ec

dE

where P is the energy-transfer probability, τD the decay time of the donor emission, QA the total absorption cross-section of the acceptor, and R the distance between the donor and the acceptor; b and c are parameters that depend on the type of energy transfer. According to this equation, the energy-transfer probability, P, is inversely proportional to the decay time, τD. In the donor−acceptor ET system, because of nonexponential decay of donor fluorescence intensity, ID(t), in the presence 7637

DOI: 10.1021/acs.iecr.5b01576 Ind. Eng. Chem. Res. 2015, 54, 7632−7639

Article

Industrial & Engineering Chemistry Research of acceptors, an average fluorescence lifetime of the donors (τD) is defined as τD =

∫0

approximately 7 Å referring to the Na2Y2B2O7:Ce3+, Tb3+, Eu3+ system,16 V = 457.82 Å3 and Z = 3 for KBaY(BO3)2 host. So the value of xs is calculated to be 85%, which is in the range of experimental Tb3+ doped content of 80−90% in the present case. Based on the discussion above, a possible scheme of Ce3+ → Tb3+ → Eu3+ energy transfer in KBaY(BO3)2 host is proposed (Figure 8). Upon NUV irradiation, electrons of Ce3+ excited from the ground state (2F5/2) to the 5d excited state then return to the lowest vibrational level of the excited state by giving out the excess energy to the surroundings. When some of these excited electrons return to the ground states (2F7/2 and 2F5/2) radiatively, the phosphor emits blue light. Others return from 5d excited states of Ce3+ to ground states via a nonradiative ET process to the 5D3 level of Tb3+ ions because of their similar energy levels, followed by nonradiative relaxation to the 5D4 level. Green emission is obtained as a result of 5D4 → 7FJ (J = 3, 4, 5, 6) transitions. Usually, the energy transfer between Tb3+ and Eu3+ ions might occur in two ways: (1) The energy transfer occurs from the f−f transition of Tb3+ ions at high energy levels to the next level f−f transition of Eu3+ excited states (i.e., excitation energy of the 5D4 level of Tb3+ transfers to the 5D1 level of Eu3+). (2) Cross-relaxation process occurs between the 5 D3 level of Tb3+ ions and 7F0 level of Eu3+ ions, which could be expressed as

∞

ID(t ) dt

where ID(t) is normalized to its initial intensity. As seen in Figure 6a, the Ce3+ lifetimes of KBYB:0.01Ce3+, yTb3+ samples + decrease with increasing Tb3 concentrations, which are 33.25, 28.99, 26.03, 24.52, 22.32, 21.73, 19.83, 18.55, and 15.47 ns for y = 0, 0.01, 0.03, 0.05, 0.10, 0.20, 0.30, 0.40, and 0.50, respectively. The result strongly demonstrates the energy transfer from the Ce3+ to Tb3+ ions. The Tb3+ lifetimes of KBYB:0.01Ce3+, yTb3+, 0.01Eu3+ samples are shown in Figure 6b. At a relative low Tb3+ content, Tb3+ lifetime reduces slightly from 2.630 ms (y = 0.10) to 2.577 ms (y = 0.30), which indicates that a Tb3+ bridge is not formed and the probability of Ce3+ → Tb3+ → Eu3+ energy transfer is low. With further increase in Tb3+ content, the decay time of Tb3+ emission decreased rapidly. The decay time of KBYB:0.01Ce3+, 0.30Tb3+, 0.01Eu3+ was 2.415 ms, whereas that of KBYB:0.01Ce3+, 0.90Tb3+, 0.01Eu3+ was 1.374 ms. The results demonstrate that the increase of Tb3+ content is beneficial for forming a Tb3+ bridge and making Tb3+ → Eu3+ energy transfer occurs with rising efficiency. The CIE coordinates of KBYB:0.01Ce 3+, yTb3+ and KBYB:0.01Ce3+, yTb3+, 0.01Eu3+ samples have been obtained by integrating their corresponding PL spectra. Table 1 and Figure 7 show the chromaticity coordinates and CIE chromaticity diagram of samples, respectively. KBYB:0.01Ce3+ has a blue emission with coordinates of (0.173, 0.087). For KBYB:0.01Ce3+, yTb3+ phosphors, tunable blue-to-green emission with CIE coordinates varying from (0.233, 0.260) to (0.341, 0.570) by changing Tb3+ content. For KBYB:0.01Ce3+, yTb3+, 0.01Eu3+ phosphors, the emitting color of the phosphor can be tuned from bluish-green (0.288, 0.410) to yellow (0.419, 0.484) and finally to the red (0.589, 0.374) with increasing content of Tb3+, corresponding to the variation of green Tb3+ emission and red Eu3+ radiation as discussed above. In order to have a vivid representation of emission colors of as-synthesized samples, digital photographs taken under a 365 nm UV lamp are also shown in Figure 7. The insufficient ET of Tb3+ → Eu3+ is responsible for the remaining Tb3+ green emission and continuing shift of CIE coordinates when Tb3+ content is below 80%. The shortened distance between Tb3+ and Eu3+ with increasing Tb3+ content is helpful for a sufficient ET and complete formation of a Tb3+ bridge. It is reported that the critical distance for Tb3+ → Eu3+ energy transfer should be close to 5−7 Å before efficient energy transfer occurs, and the corresponding energy-transfer mechanism is found to be exchange interaction.27,28 Using the empirical saturation distance (Rs) suggested by Wen and Shi,16 which acts as an assumption for sufficient ET from Tb3+ to Eu3+, the required Tb3+ content could be calculated by the following equation:

Tb3 +(5D 3) + Eu 3 +(7F0) → Tb3 +(7F5) + Eu 3 +(5D3) Tb3 +(5D 3) + Eu 3 +(7F0) → Tb3 +(7F4 ) + Eu 3 +(5D2 ) Tb3 +(5D 3) + Eu 3 +(7F0) → Tb3 +(7F3) + Eu 3 +(5D1)

In both ways, the excited energy transferred nonradiatively to the 5D0 metastable state and finally enhances the characteristic emission of Eu3+ at 592, 612, and 649 nm. 3.4. Thermal Stability. For application of phosphor in LEDs, the thermal quenching effect of the phosphors is an important technological parameter because the working temperature in the LED package could be beyond 100 °C, which has great influence on the color-rendering performance, light output, and CIE coordinates of LEDs. The temperaturedependent emission spectra of green phosphor KBYB:0.01Ce3+, 0.10Tb3+ and red phosphor KBYB:0.01Ce3+, 0.80Tb3+, 0.01Eu3+ with optimal composition excited at 364 nm were used to evaluate their stability and are depicted in Figures 9 and 10, respectively. The insets of Figures 9 and 10 show the normalized integrated PL intensity of the phosphor with temperature. When the temperature was increased up to 100 and 150 °C, the emission intensity of green phosphor KBYB:0.01Ce3+, 0.10Tb3+ dropped to 87% and 80% of its initial intensity, while that of red phosphor KBYB:0.01Ce3+, 0.80Tb3+, 0.01Eu3+ remains about 65% and 42% of PL intensity at room temperature, respectively. On the basis of the above results, it was concluded that the green phosphor KBYB:0.01Ce3+, 0.10Tb3+ has better thermal stability in comparison with that of red phosphor KBYB:0.01Ce3+, 0.80Tb3+, 0.01Eu3+.

⎡ 3V ⎤1/3 R s = 2⎢ ⎥ ⎣ 4πxsN ⎦

4. CONCLUSIONS To summarize, a series of Ce3+, Tb3+ codoped and Ce3+, Tb3+, Eu3+ tridoped KBaY(BO3)2 phosphors were synthesized via solid-state reaction. The sensitization effect of Ce3+ have been demonstrated in photoluminescent spectra of Ce3+, Tb3+

where Rs is the saturation distance between Tb3+ ions, V the volume of the unit cell, N the number of host cations in the unit cell, and xs the required Tb3+ content when Tb3+ almost completely quenches and the energy is effectively transferred to Eu3+ to maximize the emission of Eu3+. Rs is taken as 7638

DOI: 10.1021/acs.iecr.5b01576 Ind. Eng. Chem. Res. 2015, 54, 7632−7639

Article

Industrial & Engineering Chemistry Research codoped and Ce 3+ , Tb 3+ , Eu 3+ tridoped KBaY(BO 3 ) 2 phosphors. As-synthesized phosphors exhibit strong narrow line green/red emission and broadband NUV excitation. Decreased decay time of Ce3+ and Tb3+ emission confirmed the occurrence of Ce3+ → Tb3+ → Eu3+ energy transfer. Because of the long Y−Y distance (5.452(6) Å) in KBYB structure, high Tb3+ content (≥80%) is required to form a Tb3+ bridge for a complete Ce3+ → Tb3+ → Eu3+ energy transfer. Blue−green−red tunable emission color could be achieved by changing the Ce3+/Tb3+/Eu3+ ratio. The results show that Ce3+, Tb3+ codoped and Ce3+, Tb3+, Eu3+ tridoped KBaY(BO3)2 have potential to be phosphor candidates for NUV LED applications.

■

(14) Jia, Y.; Lu, W.; Guo, N.; Lu, W.; Zhao, Q.; You, H. Utilizing Tb3+ as an energy transfer bridge to connect Eu2+−Sm3+ luminescent centers: realization of efficient Sm3+ red emission under near-UV excitation. Chem. Commun. 2013, 49, 2664. (15) Xia, Z.; Zhuang, J.; Meijerink, A.; Jing, X. Host Composition Dependent Tunable Multicolor Emission in the Single-Phase Ba2(Ln1‑zTbz)(BO3)2Cl: Eu Phosphors. Dalton. Trans. 2013, 42, 6327. (16) Wen, D.; Shi, J. A novel narrow-line red emitting Na2Y2B2O7: Ce3+, Tb3+, Eu3+ phosphor with high efficiency activated by terbium chain for near-UV white LEDs. Dalton. Trans. 2013, 42, 16621. (17) Gao, J.; Song, L.; Hu, X.; Zhang, D. A buetschliite-type rareearth borate, KBaY(BO3)2. Solid State Sci. 2011, 13, 115. (18) Lian, Z.; Sun, J.; Zhang, L.; Shen, D.; Shen, G.; Wang, X.; Yan, Q. Crystal structure refinement and luminescence properties of Ce3+ singly doped and Ce3+/Mn2+ co-doped KBaY(BO3)2 for n-UV pumped white-light-emitting diodes. RSC Adv. 2013, 3, 16534. (19) Han, L.; Wang, Y.; Wang, Y.; Zhang, J.; Tao, T. Observation of efficient energy transfer from host to rare-earth ionsin KBaY(BO3)2 Tb3+ phosphor for plasma display panel. J. Alloys Compd. 2013, 551, 485. (20) Lian, Z.; Sun, J.; Ma, Z.; Zhang, L.; Shen, D.; Shen, G.; Wang, X.; Yan, Q. Synthesis, crystal structure, characterization and luminescent properties of KBaTbB2O6. J. Cryst. Growth 2014, 401, 334. (21) Shannon, R. D. Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides. Acta Crystallogr., Sect. A: Cryst. Phys., Diffr., Theor. Gen. Crystallogr. 1976, 32, 751. (22) Closs, G.; Piotrowiak, P.; MacInnis, J.; Fleming, G. Determination of long-distance intramolecular triplet energy-transfer rates. Quantitative comparison with electron transfer. J. Am. Chem. Soc. 1988, 110, 2652. (23) Setlur, A. A.; Shiang, J. J. Quantitative Analysis of Electron Transfer between Ce3+(5d1) and Yb3+/Eu3+ Ions in Y3Al5O12 and Lu2Si2O7. J. Phys. Chem. C 2010, 114, 2792. (24) Blasse, G.; Bril, A. Study of Energy Transfer from Sb3+, Bi3+, Ce3+ to Sm3+, Eu3+, Tb3+, Dy3+. J. Chem. Phys. 1967, 47, 1920. (25) Zhang, X.; Zhang, J.; Gong, M. Synthesis and luminescent properties of UV-excited thermal stable red-emitting phosphor Ba3Lu(PO4)3: Eu3+ for NUV LED. Opt. Mater. 2014, 36, 850. (26) Xia, Z.; Wu, W. Preparation and luminescence properties of Ce3+ and Ce3+/Tb3+-activated Y4Si2O7N2 phosphors. Dalton. Trans. 2013, 42, 12989. (27) Zhang, X.; Zhou, L.; Pang, Q.; Gong, M. A broadband-excited and narrow-line GdBO3: Ce3+, Tb3+, Eu3+ red phosphor with efficient Ce3+→(Tb3+)n→Eu3+ energy transfer for NUV LEDs. Opt. Mater. 2014, 36, 1112. (28) Zhang, X.; Zhou, L.; Pang, Q.; Shi, J.; Gong, M. Tunable Luminescence and Ce3+→Tb3+→ Eu3+ Energy Transfer of BroadbandExcited and Narrow Line Red Emitting Y2SiO5: Ce3+, Tb3+, Eu3+ Phosphor. J. Phys. Chem. C 2014, 118, 7591. (29) Dexter, D. L. A theory of sensitized luminescence in solids. J. Chem. Phys. 1953, 21, 836.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This research was supported by Guangxi Natural Science Foundation (Grant 2014GXNSFBA118046).

■

REFERENCES

(1) Hou, L.; Cui, S.; Fu, Z.; Wu, Z.; Fu, X.; Jeong, J. Facile template free synthesis of KLa(MoO4)2: Eu3+,Tb3+ microspheres and their multicolor tunable luminescence. Dalton Trans. 2014, 43, 5382. (2) Zhang, X.; Zhou, G.; Zhou, J.; Zhou, H.; Kong, P.; Yu, Z.; Zhan, J. Energy transfer from Bi3+ to Ho3+ triggers brilliant single green light emission in LaNbTiO6: Ho3+, Bi3+ phosphors. RSC Adv. 2014, 4, 13680. (3) Gwak, S.; Arunkumar, P.; Im, W. A New Blue-Emitting Oxohalide Phosphor Sr4OCl6: Eu2+ for Thermally Stable, Efficient White-Light-Emitting Devices under Near-UV. J. Phys. Chem. C 2014, 118, 2686. (4) Dong, X.; Zhang, J.; Zhang, X.; Hao, Z.; Luo, Y. New orange−red phosphor Sr9Sc(PO4)7: Eu3+ for NUV-LEDs application. J. Alloys Compd. 2014, 587, 493. (5) Wang, D.; Wang, Y. Luminescence properties of LaPO4: Tb3+, Me3+ (Me = Gd, Bi, Ce) under VUV excitation. Mater. Res. Bull. 2007, 42, 2163. (6) Zhu, G.; Ci, Z.; Shi, Y.; Que, M.; Wang, Q.; Wang, Y. Synthesis, crystal structure and luminescence characteristics of a novel red phosphor Ca19Mg2(PO4)14: Eu3+ for light emitting diodes and field emission displays. J. Mater. Chem. C 2013, 1, 5960. (7) Uheda, K.; Hirosaki, N.; Yamamoto, Y.; Naito, A.; Nakajima, T.; Yamamoto, H. Luminescence Properties of a Red Phosphor, CaAlSiN3: Eu2+, for White Light-Emitting Diodes. Electrochem. SolidState Lett. 2006, 9, H22. (8) Xie, R.; Li, Y.; Hirosaki, N.; Yamamoto, H. Nitride Phosphors and Solid-State Lighting; CRC Press: New York, 2011. (9) Nagaoka, Y.; Adachi, S. High Energy Transfer Efficiency in Photoluminescence of (Ce3+, Tb3+)-Codoped NaCl Green Phosphor. ECS J. Solid State Sci. Technol. 2014, 3, R43. (10) Xia, Z.; Liu, R. Tunable Blue-Green Color Emission and Energy Transfer of Ca2Al3O6F: Ce3+, Tb3+ Phosphors for Near-UV White LEDs. J. Phys. Chem. C 2012, 116, 15604. (11) Blasse, G. Optical electron transfer between metal ions and its consequences. Struct. Bonding (Berlin) 1991, 76, 153. (12) Blasse, G. Energy transfer from Ce3+ to Eu3+ in (Y, Gd)F3. Phys. Stat. Sol. A 1983, 75, K41. (13) Setlur, A. Sensitizing Eu3+ with Ce3+ and Tb3+ to Make NarrowLine Red Phosphors for Light Emitting Diodes. Electrochem. Solid-State Lett. 2012, 15, J25. 7639

DOI: 10.1021/acs.iecr.5b01576 Ind. Eng. Chem. Res. 2015, 54, 7632−7639