Photoluminescence and Energy Transfer Properties with Y+SiO4

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Photoluminescence and Energy Transfer Properties with Y+SiO4 Substituting Ba+PO4 in Ba3Y(PO4)3:Ce3+/Tb3+, Tb3+/Eu3+ Phosphors for w‑LEDs Kai Li,†,‡ Sisi Liang,†,‡ Mengmeng Shang,† Hongzhou Lian,*,† and Jun Lin*,† †

State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China ‡ University of Chinese Academy of Sciences, Beijing 100049, P. R. China S Supporting Information *

ABSTRACT: A series of Ce3+, Tb3+, Eu3+ doped Ba2Y2(PO4)2(SiO4) (BYSPO) phosphors were synthesized via the high-temperature solid-state reaction route. X-ray diffraction, high-resolution transmission electron microscopy, Fourier transform infrared, solid-state NMR, photoluminescence (PL) including temperature-dependent PL, and fluorescent decay measurements were conducted to characterize and analyze as-prepared samples. BYSPO was obtained by the substitution of Y +SiO4 for Ba+PO4 in Ba3Y(PO4)3 (BYPO). The red shift of PL emission from 375 to 401 nm occurs by comparing BYSPO:0.14Ce3+ with BYPO:0.14Ce3+ under 323 nm UV excitation. More importantly, the excitation edge can be extended from 350 to 400 nm, which makes it be excited by UV/n-UV chips (330−410 nm). Tunable emission color from blue to green can be observed under 365 nm UV excitation based on the energy transfer from Ce3+ to Tb3+ ions after codoping Tb3+ into BYSPO:0.14Ce3+. Moreover, energy transfer from Tb3+ to Eu3+ ions also can be found in BYSPO:Tb3+,Eu3+ phosphors, resulting in the tunable color from green to orange red upon 377 nm UV excitation. Energy transfer properties were demonstrated by overlap of excitation spectra, variations of emission spectra, and decay times. In addition, energy transfer mechanisms from Ce3+ to Tb3+ and Tb3+ to Eu3+ in BYSPO were also discussed in detail. Quantum yields and CIE chromatic coordinates were also presented. Generally, the results suggest their potential applications in UV/n-UV pumped LEDs.

1. INTRODUCTION In the past decades, the industry and technology of white-lightemitting diodes (w-LEDs) increase rapidly with the requirements of worldwide energy saving and environmental protection.1 Besides, it also possesses the superior merits of high efficiency, long operation time, size contract, high efficiency, etc., compared to the conventional incandescent and fluorescence lamps.2 Ultraviolet LED or near-ultraviolet (UV or n-UV LED) chips combining with blue, green, and red phosphors components to generate white light is proposed as an alternative way to replace the common fabrication method which concerns the employment of blue LED chips and YAG phosphors together.3 It can produce a high color rendering index (CRI) and good color temperature (CCT), which solve the main problems originating from lack of the red component of the common fabrication method. Therefore, searching for new phosphors matching with UV or n-UV LEDs chips and controlling the emitting colors are good research topics in the lighting field. It is well-known that Ce3+ is an excellent activator in many phosphors systems such as silicates, phosphates, aluminates, borates, and so on based on its 4f−5d allow transition, generating strong emission intensity and broad band spectra.4 It can emit different colors from ultraviolet to green and even red depending on various crystal fields it locates.5 Of course, its © XXXX American Chemical Society

broad band emission can overlap the excitation spectra of many activated ions of Eu2+, Mn2+, Tb3+, Dy3+ and so on, which makes it a good sensitizer for these ions when they are codoped into the same host and thus produce tunable emission colors via controlling the rare earth ions concnetration.6 Tb3+, a frequent green-emitting activator, was well investigated in many phosphors, owing to its dominant 5D4−7F5 transition with a general peak at 545 nm.7 However, it also can serve as a candidate sensitizer to Eu3+ ions (mainly relying on its 5D0−7F2 transition) in an appropriate matrix such as Ba3GdK(PO 4 ) 3 F:Tb 3+ ,Eu 3+ , NaLa(PO 3 ) 4 :Tb 3+ ,Eu 3+ , and NaY(MoO4)2:Tb3+,Eu3+ to bring tunable emitting color from green to red by adjusting the doped rare earth ions concentrations under UV excitation.8 Therefore, looking for Ce3+,Tb3+ and Tb3+,Eu3+ codoped phosphors is a good option in the development of novel phosphors applied in UV/n-UVpumped LEDs since there are several kinds of commercially available phosphors using Eu3+ and Tb3+ ions as activators for traditional fluorescent lamps, such as the well-known Y2O3:Eu3+, LaPO4:Ce3+,Tb3+. Recently, the photoluminescence (PL) properties of the Eu2+,Mn2+ codoped Ba3Ln(PO4)3 (Ln = Gd, Lu) series of Received: April 26, 2016

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DOI: 10.1021/acs.inorgchem.6b01040 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. (a) XRD patterns of as-prepared BYPO:0.14Ce3+, BYSPO host, and Ce3+, Tb3+ doped BYSPO samples, as well as the standard reference of Ba3La(PO4)3 compound (ICSD 80623). (b) XRD refinement for BYSPO host. (c) Crystal structure of BYSPO host, and coordinated environments of Ba/Y and P/Si atoms. (d) HRTEM of the selected area in BYSPO:0.14Ce3+,0.30Tb3+ sample; insets are the partial particle and corresponding SAED pattern.

samples have been investigated in detail by Guo et al.,9 which show their tunable color from blue-green to red upon UV excitation. Moreover, the PL properties of Ce3+ doped Ba3Ln(PO4)3 (Ln = Y, Gd, Lu) have also been reported.10 The rare earth elements have been demonstrated to be well incorporated into these compounds. Unfortunately, the maximum excitation spectrum edge in Ba3Ln(PO4)3:Ce3+ (Ln = Y, Gd, Lu) is located generally below 350 nm, which is not appropriate for the currently general UV/n-UV LEDs chips (350−410 nm). Moreover, the emitting wavelength of Ga1−xAlxN can be extended to 330 nm and ranged especially from 210 to 365 nm by changing the aluminum content in it.11 However, we notice that the emission wavelength of Eu2+ and Ce3+ can be well tuned via the different crystal field strength by the ionic substitution in recent reports.12 It shows that B2+ + E5+ − C3+ + D4+ substitution, where B are Mg, Ca, Sr, and Ba; C are Sc, Y, La, Gd, Lu or B, Al, Ga, and In; D are Si and Ge; and E are P, Mo, and W, can be a good charge balance type in phosphors to reduce the nonradiative transitions and change the emission position of rare earth ions. Therefore, we did the tentative experiments of Y+SiO4 substituting Ba+PO4 in Ba3Y(PO4)3:Ce3+ samples to change the crystal field Ce3+ locates. After some failure, the pure Ba2Y2(PO4)2(SiO4):Ce3+ were obtained. We observe that the red shift takes place in the emission spectra after the substitution under the same excitation in Ba3Y(PO4)3:Ce3+. In addition, the energy transfer properties from Ce3+ to Tb3+ and Tb3+ to Eu3+ ions have been certified and analyzed in Ba2Y2(PO4)2(SiO4) samples upon the excitation wavelength of 365 and 377 nm, respectively. Results show that tunable emission color from blue to green and green to orange red can be produced in Ba 2 Y 2 (PO 4 ) 2 -

(SiO4):Ce3+,Tb3+ and Ba2Y2(PO4)2(SiO4):Tb3+,Eu3+ upon 365 nm UV lamp excitation, respectively. The thermal quenching and quantum yields of as-prepared samples were also investigated. It illustrates that the obtained phosphors may be as potential candidate phosphors for UV/n-UV-pumped LEDs.

2. EXPERIMENTAL SECTION 2.1. Materials and Preparation. A battery of objective products Ba 3 − n Y 0 . 8 6 + n (SiO 4 ) n (PO 4 ) 3 − n :0.14Ce 3 + , Ba 2 Y 2 − x − z (PO 4 ) 2 (SiO4):xCe3+,zTb3+, and Ba2Y2−y−m(PO4)2(SiO4):yTb3+,mEu3+ (abbreviated as BYS n PO:0.14Ce 3+ , BYSPO:xCe 3+ ,zTb 3+ , and BYSPO:yTb3+,mEu3+, n = 0−1, x = 0.02−0.30, z = 0−0.42, and y = 0− 0.42, m = 0−0.30) were prepared via the common high-temperature solid-state reaction process. In a typical process, raw materials involving BaCO3 (A.R.), NH4H2PO4 (A.R.), SiO2 (A.R.), Y2O3 (99.99%), CeO2 (99.99%), Tb4O7 (99.99%), and Eu2O3 (99.99%), without any purity, were stoichiometrically weighed and blended homogeneously by grinding them in an agate mortar with appropriate addition of ethanol. The mixtures were dried for about 15 min in the oven with the fixed temperature of 60 °C, after which they were ground for 1 min, and then shifted to the crucible and the tube furnace to calcine at 1400 °C for 4 h under a 5% H2/95% N2 atmosphere for the respective Ba3Y(PO4)3:0.14Ce3+ and BYSPO:xCe3+,zTb3+ while under air condition for BYSPO:yTb3+,mEu3+ to generate the final samples. Ultimately, the samples were cooled to room temperature with the respective synthesis atmosphere and reground to powders for 1 min for the subsequent characterizations. 2.2. Measurement Characterization. X-ray diffraction (XRD) profiles of as-prepared samples were measured using a D8 Focus diffractometer equipped with the graphite-monochromatized Cu Kα radiation (λ = 0.15405 nm) at a scanning rate of 10° min−1 (0.5° min−1 for sample used for refinement) in the 2θ range from 10° to B


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Inorganic Chemistry 110°. The General Structure Analysis System (GSAS) program was used to conduct the structure refinements.13 Infrared spectra measurement was done on a VERTEX 70 Fourier transform infrared (FT-IR) spectrometer (Bruker). The solid-state NMR was collected with a conventional impulse spectrometer DSX advance (Bruker) operating with a resonance frequency of 500 MHz for 1H (B = 11.3 T). An energy dispersive spectrometer equipped with a scanning electron microscope (SEM, S-4800, Hitachi) was used to analyze the elements in BYSPO host. The selected-area electron diffraction (SAED) pattern and high-resolution transmission electron microscopy (HRTEM) were obtained using an FEI Tecnai G2 S-Twin with a field emission gun operating at 200 kV. Images were gained digitally on a Gatan multipole CCD camera. Photoluminescence (PL) measurements were conducted utilizing the Hitachi F-7000 spectrophotometer equipped with a 150 W xenon lamp as the excitation source. The fluorescent decay curves were obtained from a Lecroy Wave Runner 6100 Digital Oscilloscope (1 GHz) taking a tunable laser (pulse width = 4 ns, gate = 50 ns) as the excitation (Contimuum Sunlite OPO) source. PL quantum yields (QYs) of as-prepared phosphors were gained directly by the absolute PL quantum yield measurement system (C9920-02, Hamamatsu Photonics K. K., Japan). All the measurements above were conducted at room temperature (RT). What’s more, the temperature-dependent (298−523 K) PL spectra were measured on an Edinburgh Instruments FLSP-920 with a temperature controller.

Table 1. Crystallographic Data and Details in the Data Collection and Refinement Parameters for the BYSPO Sample sample

BYPO

BYSPO

space group symmetry a/b/c, Å V, Å3 Z α = β = γ, deg 2θ interval, deg Rwp, % Rp, % χ2

I43d cubic 10.48692(6) 1153.30(2) 4 90 10−110 3.89 2.95 3.479

I43d cubic 10.3563(2) 1110.75(5) 4 90 10−110 3.87 2.84 3.955

image of a selected part are shown in Figure 1d, in which the lattice fringes can be clearly observed. Moreover, the selectedarea electron diffraction (SAED) image of this sample is also supplied at the top right corner in Figure 1d. The interplanar distance is 0.2724 nm, corresponding to the (123) planes of BYSPO:0.14Ce3+,0.30Tb3+. These results illustrate that the single crystalline phase is formed in BYSPO:0.14Ce3+,0.30Tb3+. FT-IR and solid NMR spectra of representative samples are supplied in Figure 2 to demonstrate the formations of Si-O bands and [SiO4]4− tetrahedra in BYSPO. As seen in Figure 2a, the peaks at 548, 974, and 1044 cm−1 are ascribed to the symmetric stretching mode of (PO4)3− units in BYPO:0.14Ce3+ because the IR absorption bands of (PO4)3− generally locate at 1120−940 and 650−540 cm−1 scopes.14 Another obvious peak at 3435 cm−1 is assigned to OH− vibration resulting from the cover water on the surface of the as-prepared sample in air condition. It is very different from BYPO:0.14Ce3+ in that the profile varies and two extra peaks at 871 and 918 cm−1 are produced evidently in the BYSPO:0.14Ce3+ sample compared to BYPO:0.14Ce3+, which can be approximately assigned to the asymmetric Si-O stretching modes of the SiO4 tetrahedron [900−1100 cm−1 refers to ν(SiO4) reported in the previous literature15]. The 29Si-MAS NMR and 31P-MAS NMR spectra of the BYSPO host show two obvious resonances at around −99.4 and −6.25 ppm as shown in Figure 2b,c, respectively. These two main positions further testify the existences of the SiO4 and PO4 tetrahedrons in the BYSPO host. These results can illustrate that Si have been introduced into BYPO to form SiO4 tetrahedrons. 3.2. Photoluminescence Properties. Figure 3a shows the comparison of PL emission and excitation spectra for BYPO:0.14Ce3+ and BYSPO:0.14Ce3+ samples. It can be seen that the emission spectrum of BYPO:0.14Ce3+ consists of a broad band with the range from 330 to 525 nm peaking at 375 nm under 323 nm excitation, while it involves the emission wavelength ranging from 330 to 600 nm centered at 401 nm for BYSPO:0.14Ce3+. This illustrates that a red shift has taken place when Y+SiO4 substitutes Ba+PO4 in the Ba3 Y(PO4)3:0.14Ce3+ sample. More attention is paid to the variation of excitation spectra. It is found that the excitation spectrum of BYSPO:0.14Ce3+ produces an extra band peaking at 356 nm monitored at 401 nm besides the similar band with BYPO:0.14Ce3+ monitored at 375 nm. Therefore, the excitation spectrum edge can be extended from 350 nm for BYPO:0.14Ce3+ to about 400 nm for BYSPO:0.14Ce3+, which makes it be more appropriately excited by current UV/n-UV (350−410 nm) chips. These phenomena can be explained by

3. RESULTS AND DISCUSSION 3.1. Phase Recognition. The identifications of phase composition and purity of representative samples were conducted by XRD powder patterns and GASA Rietveld refinement at room temperature. As presented in Figure 1a, the XRD profiles of BYSPO host, Ce3+ doped BYPO, and Ce3+, Tb3+, Eu3+ doped BYSPO were all well indexed to the pure Ba3La(PO4)3 compound (ICSD 80623) which is isostructural with Ba3Y(PO4)3. Therefore, the deviation of diffraction positions originated from the different atom radii of Y and La, and the substitution of Ba+PO4 by Y+SiO4, as well as the introductions of rare earth ions in these samples. Figure S1 and Figure 1b show the Rietveld refinements of representative BYPO and BYSPO hosts. Since the structure of Ba3Y(PO4)3 has not ever been known, the original structure model and crystallographic data for the refinement were referred to the Ba3La(PO4)3 compound (ICSD 80623). As shown in Figure S1 and Figure 1b, the black crosses and olive solid line refer to the experimental and calculated patterns, respectively. The red short vertical lines represent the Bragg diffraction positions of the calculated pattern. The differences between the experimental and calculated results are signed by the dark cyan line at the bottom. All the related atomic positions, fraction factors, and thermal vibrational parameters were converged and refined. The summarized residual factors and cell parameters are listed in Table 1 in detail, which indicate that the refined results are convincible. The structures of BYPO and BYSPO compounds (along a, b axes) illustrate that they crystallize in a cubic system with the space group I43d. The Ba and Y atoms (green balls) occupy the same site with equal quantity in Figure 1c and six coordinated oxygen atoms (red balls) around. Si would like to occupy P atoms sites to form a [SiO4] tetrahedron besides the [PO4] tetrahedron, and one-third of P is substituted by Si. The Ce3+, Tb3+, and Eu3+ were commonly considered to be incorporated into Ba/Y sites based on their similar ionic radii of Y3+ (coordination number (CN) = 6, r = 0.90 Å) and Ce3+ (CN = 6, r = 1.01 Å), Tb3+ (CN = 6, r = 0.92 Å), and Eu3+ (CN = 6, r = 0.95 Å). The low-resolution TEM image of one particle of BYSPO:0.14Ce3+,0.30Tb3+ and the corresponding HRTEM C


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Inorganic Chemistry

Figure 2. (a) FT-IR spectra of as-prepared BYPO:0.14Ce3+ and BYSPO:0.14Ce3+ phosphors. (b) 29Si-MAS NMR and (c) 31P-MAS NMR spectra of BYPSO host.

Figure 3. (a) PL emission spectra of BYSnPO:0.14Ce3+ (n = 0, 0.25, 0.5, 0.75, and 1) and excitation spectra of BYPO:0.14Ce3+ and BYSPO:0.14Ce3+. (b) PL emission and excitation spectra of BYSPO:0.14Ce3+ phosphor. (c) Variation of emission spectra as a function of Ce3+ concentration x in BYSPO:xCe3+ (x = 0.02−0.30); inset is the variation of emission intensity with different x. (d) Linear fitting of log(x) versus log(I/x) in various BYSPO:xCe3+ phosphors beyond the concentration quenching (x ≥ 0.14).

the different crystal fields and the enhancement of the crystal field Ce3+ around. Since the ionic radius of Y3+ (CN = 6, r = 0.90 Å) is much smaller than that of Ba2+ (CN = 6, r = 1.36 Å), together with the bigger positive charge number, the substitution of Y for Ba will result in the contraction of the BaO6 octahedron. Besides, the representative Y−O bond lengths for the BYPO host and BYPSO host show the contraction of the YO6 octahedron after substitution in Table 2. Moreover, the substitution of Si for P generates the inflation of the PO4 tetrahedron because of the smaller positive charge and bigger ionic radius. Therefore, the crystal field for Ce3+ occupying original Y sites enhances, resulting in the red shift of

Table 2. Selected Y−O Bond Lengths for BYPO and BYSPO Samples Y−O bond length (Å) average length (Å)

BYPO host

BYPSO host

2.775(1) × 3 2.753(1) × 3 2.764(1)

2.567(4) × 3 2.672(5) × 3 2.620(5)

emission. The generation of the new YO6 octahedron originated from the substitution of Y for Ba is different from the original YO6 in BYPO:0.14Ce3+ because of the substitution of Si for P simultaneously, which may arouse another excitation D


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Inorganic Chemistry band in BYSPO:0.14Ce3+. As two different Ce3+ sites exist in BYSPO:0.14Ce3+, the asymmetric emission spectrum under 326 nm excitation of BYSPO:0.14Ce3+ can be deconvoluted into two Gaussian bands with the emission peaks at 398 and 435 nm in Figure 3b. In addition, we can see that the sample also can be effectively excited with the wavelengths 356 and 365 nm. Figure 3c displays the variation of emission spectra upon 365 nm excitation. A minor red shift can be observed with the increase of Ce3+ doping concentration in BYSPO:Ce3+, which may be attributed to the bigger Ce3+ entering into smaller Y3+ sites, resulting in the closer distance between Ce3+−O2− with increasing Ce3+ content, and thus causing a larger splitting of the 5d level and exhibiting a red shift in the emission spectra. The emission intensity of BYSPO:xCe3+ increases monotonously until x = 0.14, as depicted in the inset of Figure 3c, and falls subsequently with further Ce3+ content due to the familiar concentration quenching effect. Therefore, the Ce3+ content was fixed of 0.14 for the subsequent Ce3+,Tb3+ codoped BYSPO samples. The critical distance (Rc) between activator Ce3+ is related to the energy transfer mechanism between Ce3+; therefore, it should be calculated first, as expressed below16 ⎡ 3V ⎤1/3 R c ≈ 2⎢ ⎥ ⎣ 4πXcN ⎦

(1) Figure 4. PL excitation and emission spectra of BYSPO:0.24Tb3+ (a) and BYSPO:0.08Eu3+ (b) samples.

where N and V are the number of host cations and volume of the unit cell in a unit cell, respectively, and Xc corresponds to the critical concentration of dopant ions Ce3+ here. As listed in Table 1 for the BYSPO host, the N = 4, V = 1110.75 Å, and Xc equals to 0.14. Accordingly, the Rc is approximately determined to be 15.59 Å by calculation using these parameters. The value here gives the exclusion of exchange interaction because the exchange interaction is predominant only for about 5 Å. Commonly, the most possible way for the nonradiate energy transfer between Ce3+ in BYSPO is electric multipolar interactions including electric dipole−dipole, dipole−quadrupole, and quadrupole−quadrupole. To determine which one is the most possible in this condition, we can analyze the θ value in the following equation17 I = [1 + β(x)θ /3 ]−1 x

excitation spectrum of BYSPO:0.24Tb3+ monitored at the most intense peak at 550 nm shows a broad band, resulting from the Tb3+ 4f−5d transition, with the peak at 247 nm and several characteristic excitation lines at 285, 320, 353, 377, and 485 nm due to Tb3+ 4f−4f transitions, corresponding to its 7F6−5H6, 7 F6−5D0, 7F6−5D2, 7F6−5G6, and 7F6−5D4, respectively. As to BYSPO:0.08Eu3+, the excitation spectrum monitored at 616 nm presents a broad band centered at 262 nm that originated from Eu3+−O2− charge transfer transition and several emission lines between 300 and 500 nm at 321, 363, 383, 395, 414, and 466 nm derived from 7F0−5H3, 7F0−5D4, 7F0−5G2, 7F0−5L6, 7 F0−5D3, and 7F0−5D2 transitions, respectively, among which the excitation intensity at 395 nm is the strongest. Upon 395 nm excitation, the emission spectrum displays its many characteristic lines at about 581, 592, 616, 657, and 706 nm, which are assigned to the 5D0 → 7FJ (J=0,1,2,3,4) transitions. Moreover, we can observe that the intensity of the 5D0 → 7F1 magnetic-dipole transition (592 nm) is much lower than that of the 5D0 → 7F2 electric-dipole transition (616 nm). It illustrates that Eu3+ ions mainly occupy the inversion symmetry sites in the BYSPO compound. Figure S2 supplies the variations of emission spectra of BYSPO:yTb3+ prepared under air condition and the corresponding intensities as a function of Tb3+ concentration under 247 and 377 nm excitation. Upon 247 nm excitation, all the emission spectra of BYSPO:Tb3+ consist of both 5D3 → 7 FJ (J=6,5,4,3,2) and 5D4 → 7FJ (J=6,5,4,3) transitions, as depicted above. It is known to all that the energy gap between two 5D3 and 5D4 energy levels is close to that between the 7F0 and 7F6 ones, which often produces the energy transfer properties of 5 D3 (Tb3+) + 7F6 (Tb3+) → 5D4 (Tb3+) + 7F0 (Tb3+).18 Therefore, the emission intensities of both 5D3 and 5D4 increase with the increase of Tb3+ concentration (y ≤ 0.02), while the emission intensity of 5D3 decreases with further Tb3+ concentration (y > 0.02) owing to the energy transfer

(2)

where x and I represent the concentration and emission intensity of the activator ion such as Ce3+ here, respectively, and β is a constant for the certain matrix under the same excitation condition. θ = 6, 8, and 10 correspond to electric dipole− dipole, dipole−quadrupole, and quadrupole−quadrupole interactions, respectively. Figure 3d presents the curve of log(I/x) versus log(x) in BYSPO:Ce3+ phosphors beyond the quenching content of Ce3+. It can be fitted with a straight line and corresponding slope of −1.742 = −θ/3; as a consequence, the θ = 5.226, approximately as 6, indicating that the energy transfer between Ce3+ occurs with dipole−dipole interaction in BYSPO:Ce3+ phosphors. Figure 4a,b shows the PL emission and excitation spectra of Tb3+ and Eu3+ singly doped BYSPO samples, respectively. As illustrated in Figure 4a, the emission spectrum of BYSPO:0.24Tb3+ under 247 nm radiation presents many characteristic peaks at 382, 418, 439, 461, 478 and 491, 550, 586, 625 nm, which correspond to Tb3+ 5D3 → 7FJ (J=6,5,4,3,2) and 5D4 → 7FJ (J=6,5,4,3,), respectively. It also can be well excited under 377 nm UV light excitation, whose emission spectrum is similar to that excited at 247 nm. We can observe that the E


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Inorganic Chemistry properties above. As to 5D4, the emission intensity of it increases monotonously until y = 0.30, beyond which the intensity begins to fall due to the common concentration quenching effect. Upon 377 nm, the BYSPO:yTb3+ samples mainly include the 5D4 → 7FJ (J=6,5,4,3) transitions, the emission intensity also increases with increasing Tb3+ concentration, and the maximum emission intensity occurs when y = 0.30. Therefore, the subsequent Tb3+ concentration is fixed at 0.30 in Tb3+,Eu3+ codoped BYSPO samples. In the BYSPO:0.14Ce3+,0.24Tb3+ sample, the emission spectrum contains both Ce3+ and Tb3+ emission bands upon 365 nm excitation in Figure 5. Monitored at 550 nm (Tb3+

emission), the profile of the excitation spectrum is similar to that monitored at 409 nm (Ce3+ emission) except for the Tb3+ 4f−5d transition. This phenomenon implies that the Tb3+ emission energy mainly originated from Ce3+; that is to say, energy transfer from Ce3+ to Tb3+ ions in BYSPO takes place under UV/n-UV excitation. Variations of emission spectra excited at 365 nm of BYSPO:0.14Ce 3+ ,zTb 3+ (z = 0−0.42) and BYSPO:xCe3+,0.24Tb3+ (x = 0.02−0.30) are exhibited in Figure 6. As depicted in Figure 6a,c, the emission spectra of all the Ce3+,Tb3+ codoped samples contain both Ce3+ and Tb3+ emission bands. In Figure 6a, the emission intensity of Ce3+ decreases monotonously with the increase of Tb3+ concentration from z = 0 to 0.42, while the Tb3+ emission intensity increases to a maximum at z = 0.30, then decreases owing to the concentration quenching effect in Figure 6b. This appearance indicates that the energy transfer from Ce3+ to Tb3+ ions can occur in current excitation condition in BYSPO:Ce3+,Tb3+. Therefore, the emission color can be tuned from blue to green with increasing Tb3+ concentration in BYSPO:0.14Ce3+,zTb3+ samples under 365 nm excitation. In BYSPO:xCe3+,0.24Tb3+ (x = 0.02−0.30) samples, we can find that both emission intensities of Ce3+ and Tb3+ first increase in Figure 6c,d because the increase of Ce3+ concentration results in more sensitizers transferring their energy to Tb3+ ions until x = 0.10, and then the emission intensity of Ce3+ decreases because of the concentration quenching effect with further Ce3+ concentration. However, the energy transfer process from Ce3+ to Tb 3+ saturates until x = 0.14, together with Ce 3+

Figure 5. PL emission and excitation spectra of BYSPO:0.14Ce3+,0.24Tb3+ phosphor.

Figure 6. (a) Dependences of emission spectra and corresponding intensities of Ce3+ and Tb3+ (b) on Tb3+ concentration z in BYSPO:0.14Ce3+,zTb3+ (z = 0−0.42). (c) Dependences of emission spectra and corresponding intensities of Ce3+ and Tb3+ (d) on Ce3+ concentration x in BYSPO:xCe3+,0.24Tb3+ (x = 0.02−0.30). F


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Inorganic Chemistry concentration quenching, leading to the decrease of Tb3+ emission intensity with further Ce3+ concentration. Figure 7 shows the PL excitation and emission spectra of the representative 0.30Tb3+,0.08Eu3+ phosphor. It can be well

Figure 7. PL emission and excitation spectra of BYSPO:0.30Tb3+,0.08Eu3+ phosphor.

observed that the emission spectrum consists of both Tb3+ and Eu3+ characteristic emission lines with the range from 400 to 720 nm upon 377 nm excitation. Monitored at 550 nm (Tb3+ emission), the excitation spectrum displays the Tb3+ characteristic profile similar to the Tb3+ singly doped BYSPO sample. However, the excitation spectrum monitored at 616 nm (Eu3+ emission) includes not only the Eu3+ characteristic excitation profile (including a peak signed at 363 nm in Figure 7) similar to the Eu3+ singly doped sample but also two obvious excitation lines at 353 and 485 nm derived from Tb3+. Therefore, we can deduce that the partial energy of Eu3+ originated from Tb3+ under 377 nm; in other words, the energy transfer from Tb3+ to Eu3+ can be proceeded in the BYSPO:Tb3+,Eu3+ phosphors under the current excitation condition. A series of samples with different Eu3+ concentrations in BYSPO:0.30Tb3+,mEu3+ (m = 0−0.30) have been prepared to recognize the energy transfer effect from Tb3+ to Eu3+ ions. The variations of emission spectra and corresponding intensities of BYSPO:0.30Tb3+,mEu3+ are plotted in Figure 8a,b, respectively. We can see that the emission profiles of Tb3+,Eu3+ codoped BYSPO samples own both Tb3+ and Eu3+ emission lines under 377 nm excitation. With increasing Eu3+ concentration, the emission intensity of Tb3+ decreases monotonously contrary to the Eu3+ emission intensity in the current concentration interval. This phenomenon can give us a confirmation that energy transfer from Tb3+ to Eu3+ also generates upon the Tb3+ characteristic excitation range. In order to further demonstrate the energy transfer phenomena from Ce3+ to Tb3+ in BYSPO:Ce3+,Tb3+ and from Tb3+ to Eu3+ in BYSPO:Tb3+,Eu3+, the fluorescent decay curves of the respective Ce3+ and Tb3+ ions have been obtained. Decay curves and the correspondingly calculated decay times of Ce3+ in BYSPO:0.14Ce3+,zTb3+ monitored at 409 nm with 365 nm excitation and Tb3+ in BYSPO:0.30Tb3+,mEu3+ monitored at 550 nm with 377 nm excitation are presented in Figure 9a,b, respectively. The decay curves of Ce3+ can be fitted well with a single exponential function, as expressed below19 I = A exp( −t /τ )

Figure 8. (a) Dependence of emission spectra and corresponding intensities of Tb3+ and Eu3+ (b) on Eu3+ concentration m in BYSPO:0.30Tb3+,mEu3+ phosphors (m = 0−0.30).

Figure 9. (a) Fluorescent decay curves of Ce3+ in BYSPO:0.14Ce3+,zTb3+ monitored at 409 nm with 365 nm excitation and (b) Tb3+ in BYSPO:0.30Tb3+,mEu3+ monitored at 550 nm with 377 nm excitation.

where I and t are the corresponding luminescent intensity and time of the certain sample, A is a constant, and τ is the lifetime for the exponential component. According to the eq 3, the decay times are approximately estimated to be 47.7, 42.3, 37.9, and 29.3 ns corresponding to z = 0, 0.06, 0.10, and 0.30 in Figure 9a, respectively. It is clearly observed that the decay times of Ce3+ drops with the increase of Tb3+ concentration z in BYSPO:014Ce3+,zTb3+. As for Tb3+ in BYSPO:0.30Tb3+,mEu3+, the decay curves of Tb3+ are not well fitted with any exponential function; therefore, it can be simply calculated as follows20

(3) G


Article

Inorganic Chemistry τ=

∫0

Table 3. Variations of CIE Chromaticity Coordinates (x, y) and Quantum Yields (QYs) for BYSPO:0.14Ce3+,zTb3+ Excited at 365 nm UV and BYSPO:0.30Tb3+,mEu3+ Phosphors Excited at 377 nm UV

∞

I (t ) d t

(4)

where τ and I(t) are the calculated lifetime value and corresponding normalized intensity of the emission curves, respectively. Accordingly, the lifetimes of Tb3+ in BYSPO:0.30Tb3+,mEu3+ are calculated to be 1.113, 0.978, 0.892, and 0.651 ms in Figure 9b, corresponding to Eu 3+ concentrations m = 0, 0.02, 0.08, and 0.24, respectively. Similarly, the decay lifetime decreases with constantly increasing Eu3+ concentration m in BYSPO:0.30Tb3+,mEu3+. Results of fluorescent decay lifetimes strongly certify the energy transfers from Ce3+ to Tb3+ ions in BYSPO:Ce3+,Tb3+ and from Tb3+ to Eu3+ ions in BYSPO:Tb3+,Eu3+ under UV/n-UV excitation. In addition, the energy transfer efficiency (ηT) from Ce3+ to Tb3+ and Tb3+ to Eu3+ in BYSPO can be approximately calculated using the variations of emission intensities of Ce3+ and Tb3+ with the following equation21 I ηT = 1 − S IS0 (5)

sample no.

Tb3+ concentration (z)

CIE coordinates (x, y)

QY (%)

1 2 3 4 5 6 7 8 9 sample no.

0 0.02 0.06 0.10 0.14 0.18 0.24 0.30 0.42 concentration (m)

(0.157, 0.080) (0.164, 0.107) (0.175, 0.154) (0.186, 0.204) (0.192, 0.223) (0.201, 0.262) (0.207, 0.285) (0.225, 0.357) (0.227, 0.367) CIE coordinates (x, y)

69.7 83.2 84.3 75.3 89.2 85.0 75.3 58.9 59.8 QY (%)

10 11 12 13 14 15 16 17

where ηT corresponds to the energy transfer efficiency, and IS0 and IS refer to the corresponding luminescence intensities of Ce3+ and Tb3+ ions with the absence and presence of Tb3+ and Eu3+ ions in BYSPO:Ce 3+,Tb3+ and BYSPO:Tb3+,Eu 3+, respectively. As a result, the energy transfer efficiencies from both Ce3+ to Tb3+ and Tb3+ to Eu3+ increase gradually with increasing Tb3+ and Eu3+ contents in BYSPO:0.14Ce3+,zTb3+ (a) and in BYSPO:0.30Tb3+,mEu3+ (b), as plotted in Figure 10,

Eu3+

0 0.02 0.04 0.08 0.12 0.16 0.24 0.30

(0.272, (0.356, (0.387, (0.434, (0.472, (0.500, (0.532, (0.545,

0.527) 0.476) 0.456) 0.419) 0.393) 0.382) 0.362) 0.357)

57.3 77.5 65.3 60.2 61.0 64.5 60.3 58.0

BYSPO:0.30Tb3+,mEu3+, the CIE chromaticity coordinate ranges from point 10 (0.272, 0.527) to 17 (0.545, 0.357), corresponding to green and orange red regions. These CIE chromaticity coordinates have been clearly signed in the middle of Figure 11. Left and right in Figure 11 are the simple energy level diagrams for Ce3+ and Tb3+, and Tb3+ and Eu3+, respectively, which are used to better describe the energy transfer processes from Ce3+ to Tb3+ and Tb3+ to Eu3+ in BYSPO. In general, the nonradiation energy transfer process proceeds from a sensitizer to an acceptor often via exchange interaction and electric multipolar interactions compromising electric dipole−dipole, dipole−quadrupole, and quadrupole−quadrupole interactions. To determine which one predominates the energy transfer mechanism, Reisfeld’s approximation and Dexter’s energy transfer formula is utilized to analyze, which can be expressed below:22 ηS0 ∝ C α /3 ηS (6) Herein, ηS0 and ηS are the luminescence quantum efficiencies of Ce3+ and Tb3+ ions with the absence and presence of Tb3+ and Eu3+ ions in BYSPO:Ce3+,Tb 3+ and BYSPO:Tb3+,Eu3+, respectively. C is the total contents of the respective Ce3+ and Tb3+or Tb3+ and Eu3+. α = 3, 6, 8, and 10 refer to the corresponding exchange interaction, electric dipole−dipole, dipole−quadrupole, and quadrupole−quadrupole interactions, respectively. Since it is difficult to know ηS0/ηS, we adopt the IS0/IS to replace the ηS0/ηS. Thus, the formula is presented as below: IS0 ∝ C α /3 IS (7)

Figure 10. Dependence of energy transfer efficiencies (ηT) on Tb3+ (z) and Eu3+ (m) concentrations in BYSPO:0.14Ce3+,zTb3+ (a) and BYSPO:0.30Tb3+,mEu3+ (b) phosphors.

respectively. The maximum values from Ce3+ to Tb3+ and Tb3+ to Eu3+ are approximately 70.8% and 86.1% in current concentration intervals, respectively. Besides, the values of CIE chromaticity coordinates (x, y) and absolute quantum yields (QYs) for BYSPO:0.14Ce3+,zTb3+ excited at 365 nm UV and BYSPO:0.30Tb3+,mEu3+ phosphors excited at 377 nm UV are listed in Table 3. The maximum QY are 89.2% (95% for standard YAG:Ce as the reference) and 77.5% corresponding to z = 0.14 and m = 0.02, respectively. The CIE chromaticity coordinates (x, y) for BYSPO:0.14Ce3+,zTb3+ excited at 365 nm UV vary from point 1 (0.157, 0.080) to 9 (0.227, 0.367) with corresponding z = 0 to z = 0.42, thereof the emission color can tune from blue to green area with increasing z. As for

Herein, the IS and IS0 stand for the luminescence intensities of the Ce3+ and Tb3+ ions with and without the corresponding Tb3+ and Eu3+ ions in BYSPO:Ce3+,Tb3+ and BYSPO:Tb3+,Eu3+, respectively. As is depicted in Figure 12 a,b, the best H


Article

Inorganic Chemistry

Figure 11. Simple energy level diagrams of energy transfer process from Ce3+ to Tb3+ ions and Tb3+ to Eu3+ ions in BYSPO:Ce3+,Tb3+ and BYSPO:Tb3+,Eu3+ are on the left and right, respectively. CIE chromaticity coordination for BYSPO:Ce3+,Tb3+ (1−9) under 365 nm excitation and BYSPO:Tb3+,Eu3+ (10−17) under 377 nm excitation is in the middle of picture. The digital luminescence photographs of the corresponding phosphors excited under a 365 nm UV lamp are shown on the right of picture.

radiative decay time of the sensitizer, E is the energy involved in the transfer (in eV), ∫ f S(E)fA(E)/E4 dE refers to the spectral overlap between the normalized shapes of the sensitizer Ce3+ emission f S(E) and the activator Tb3+ excitation fA(E). By taking PSAτS = 1, corresponding to the equal probability of energy transfer and radiative emission of the sensitizer, the critical distance Rc value from the sensitizer to the activator can be calculated using following equation R c6 = 3.024 × 1012fd

∫

fS (E)fA (E) E4

dE

(9)

where fd of the Eu3+ transition is approximately 0.02, ∫ f S(E) fA(E)/E4 dE = 4.46 × 10−5 eV−1; accordingly, the Rc = 11.8 Å, which coincides well with that obtained using the concentration quenching method, 11.18 Å (eq 1; the total concentrations of Tb3+ and Eu3+ Xc are equal to 0.38 when Tb3+ emission intensity is half of the initial one). This demonstrates that the dipole−dipole interaction is responsible for the energy transfer mechanism from Tb3+ to Eu3+ ions in BYSPO:Tb3+,Eu3+. Considering dipole−quadrupole interactions in BYSPO:Ce3+,Tb3+ samples here, the probability of energy transfer from Ce3+ to Tb3+ ions can be calculated according to formula below: 3+

3+

6/3

PSA(dq) = 3.024 × 1012

8/3

Figure 12. Dependence of IS0/IS of Ce and Tb on C, C , C , and C10/3 in BYSPO:Ce3+,Tb3+ (a) and BYSPO:Tb3+,Eu3+ (b), respectively.

4.8 × 10−16fd 6

R τS

∫

fS (E)fA (E) E4

8

R τS

∫

fS (E)fA (E) E4

dE (10)

By taking PSAτS = 1, the formula can be expressed below:

linear relationships are observed as α = 6 and 8, respectively, in BYSPO:Ce3+,Tb3+ and BYSPO:Tb3+,Eu3+, indicating that electric dipole−quadrupole and dipole−dipole interactions would contribute to the energy transfer mechanisms from Ce3+ to Tb3+ in BYSPO:Ce3+,Tb3+ and Tb3+ to Eu3+ ions in BYSPO:Tb3+,Eu3+, respectively. In the case of dipole−dipole interactions in BYSPO:Tb3+,Eu3+ samples here, the probability of energy transfer from the sensitizer Tb3+ to the acceptor Eu3+ can be expressed by the following equation23 PSA(dd) = 0.63 × 1028

λS2fd

R c8 = 3.024 × 1012λS2fq

∫

fS (E)fA (E) E4

dE

(11)

λS = 4090 Å (in angstroms) is the wavelength position of the sensitizer’s emission. fq is the oscillator strength of the Tb3+ quadrupole transition, which has not been obtained up to now unfortunately. However, the ratio fq/fd is proposed to be about 10−3−10−2 by Verstegen et al, while fd (oscillator strength of the Tb3+ electric dipole transition) is in the order of 10−6. After utilizing these parameters and calculated spectral overlap part ∫ f S(E)fA(E)/E4 dE = 8.11 × 10−4 eV−1, we can determine that the Rc is approximately 8.95−11.93 Å, which is also consistent with 11.38 Å obtained by the concentration quenching method (eq 1; the total concentrations of Ce3+ and Tb3+ Xc are equal to 0.36 when Ce3+ emission intensity is half of the initial one). Therefore, it is reasonable that dipole−quadrupole interactions dominate the energy transfer process from Ce3+ to Tb3+ ions in BYSPO:Ce3+,Tb3+ phosphors.

dE (8)

where fd refers to the oscillator intensity of the dipole−dipole electrical absorption for the activator, R represents the distance between the activator and the sensitizer, τS corresponds to the I


Article

Inorganic Chemistry

Figure 13. (a, c) The dependences of emission spectra of BYSPO:0.14Ce3+,0.30Tb3+ (λex = 365 nm) and BYSPO:0.30Tb3+,0.08Eu3+ (λex = 377 nm) on temperature; insets are the corresponding emission intensities of Ce3+ and Tb3+, Tb3+ and Eu3+, respectively. (b, d) The linear relationships of ln(I0/I − 1) versus 1/kT activation energy graph for thermal quenching of BYSPO:0.14Ce3+,0.30Tb3+ and BYSPO:0.30Tb3+,0.08Eu3+ samples; insets are the integrated intensities of corresponding emissions, respectively.

3.3. Photoluminescence Thermal Quenching. It is accepted that thermal quenching properties are a vital parameter for phosphors applied in LEDs; therefore, we do the photoluminescence measurement upon 365 nm excitation and 377 nm excitation with the variation of temperature to explore thermal quenching properties of representative BYSPO:0.14Ce3+,0.30Tb3+ and BYSPO:0.30Tb 3+,0.08Eu3+ phosphors, respectively. As depicted in Figure 13 a, the variations of emission intensity for Ce3+ and Tb3+ are different from each other, which illustrates that the decrease of Ce3+ is much faster than that of Tb3+ in BYSPO:Ce3+,Tb3+. The emission intensities of Ce3+ and Tb3+ in BYSPO:Ce3+,Tb3+ keep about 74.8% and 80.1% of their initial values, respectively. Similarly, we can observe that Eu3+ decreases faster than that of Tb3+ in BYSPO:Tb3+, Eu3+ phosphors in Figure 13c. The emission intensities of Tb3+ and Eu3+ in BYSPO:Tb3+,Eu3+ keep about 82.2% and 69.2% of their initial values, respectively. On one hand, the emission intensities of Ce3+, Tb3+, and Eu3+ generally decrease with increasing operated temperature, which is ascribed to the thermal quenching via the thermal activation through the crossing point between the ground and the excited states. On the other hand, the energy transfer effect from Ce3+ to Tb3+ or Tb3+ to Eu3+ ions in BYSPO will affect the emission intensities of Ce3+, Tb3+, and Eu3+ ions with increasing temperature. Therefore, the combination of these two factors results in the different rates of decays of Ce3+ and Tb3+ in BYSPO:Ce3+,Tb3+ and Tb3+ and Eu3+ in BYSPO:Tb3+,Eu3+. Activation energy (Ea) is a good parameter to reflect the thermal stability of luminescence in phosphors. It can be

obtained according to the analysis of the Arrhenius formula below24

⎛I ⎞ E ln⎜ 0 ⎟ = ln A − a ⎝I⎠ kT

(12)

where A and k are a constant and the Boltzmann constant (8.626 × 10−5 eV), respectively. T and Ea stand for the certain temperature (K) and estimated activation energy. I0 and I are the integrated intensities at room temperature (25 °C) and differently operated temperatures of emission spectra, respectively. As illustrated in Figure 13b,d, the linear fittings of the ln(I0/I − 1) versus 1/kT activation energy graph for thermal quenching of BYSPO:0.14Ce3+,0.30Tb3+ and BYSPO:0.30Tb3+,0.08Eu3+ are conducted, in which the slopes are −0.259 and −0.239, respectively. As a consequence, the Ea are 0.259 and 0.239 eV for BYSPO:0.14Ce3+,0.30Tb3+ and BYSPO:0.30Tb3+,0.08Eu3+, respectively. The activation energies are close to those of well-known phosphors of SrSiO2N2:Eu2+ (0.24 eV) and SrSi2O4:Eu2+ (0.22 eV). In addition, the variations of calculated CIE chromatic coordinates of BYSPO:0.14Ce3+,0.30Tb3+ and BYSPO:0.30Tb3+,0.08Eu3+ with increasing temperature are displayed in Figure 14, which shows that their original values are different from those in Figure 11 because of different instrumentations between PL and temperaturedependent PL spectra. We can see that the CIE chromatic coordinates change from (0.313, 0.461) at 25 °C to (3.24, 0.481) at 250 °C for BYSPO:0.14Ce3+, 0.30Tb3+ and from (0.434, 0.419) to (0.362, 0.387). It is evitable that the CIE chromatic coordinates vary with increasing temperature due to J


Article

Inorganic Chemistry Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project is financially supported by the National Natural Science Foundation of China (NSFC Grants 91433110, 51472234), the Natural Science Foundation of Jilin Province (20150520029JH), the National Basic Research Program of China (Grants 2014CB643803), and Joint Funds of the National Natural Science Foundation of China (Grant U1301242).

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Figure 14. Variations of calculated CIE chromatic coordinates of BYSPO:0.14Ce3+,0.30Tb3+ and BYSPO:0.30Tb3+,0.08Eu3+ with increasing temperature.

the different rates of decreases of emission components of Ce3+, Tb3+, and Eu3+ in phosphors.

4. CONCLUSIONS In summary, a series of Ce3+, Tb3+, Eu3+ doped Ba2Y2(PO4)2(SiO4) (BYSPO) phosphors were prepared via the hightemperature solid-state reaction route. The phase of Ba2Y2(PO4)2(SiO4) was obtained via the substitution of Y+SiO4 for Ba+PO4 in Ba3Y(PO4)3 (BYPO), which produces the red shift of the emission spectrum from BYPO:0.14Ce3+ to BYSPO:0.14Ce3+ under the same excitation conditions. Moreover, the attracted tip is the prolongation of the excitation edge from 350 to 400 nm, matching with UV/n-UV chips (330−410 nm). After being introduced Tb3+ with Ce3+ together in BYSPO, the energy transfer from Ce3+ to Tb3+ ions can be observed, resulting in the tunable color from blue to green upon 365 nm excitation via controlling the doped ions’ concentration. In addition, the Tb3+ and Eu3+ codoped BYSPO phosphors can present the tunable color from green to orange red upon 377 nm excitation based on the energy transfer from Tb3+ to Eu3+ ions. Energy transfer mechanisms from Ce3+ to Tb3+ and Tb3+ to Eu3+ in BYSPO are demonstrated to be dipole−quadrupole and dipole−dipole, respectively. Relatively high quantum yields and good thermal stability were also observed in as-prepared samples. These results indicate that they can serve as the candidates of phosphors for UV/n-UV-pumped LEDs.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b01040. XRD refinement of BYPO host (Figure S1), the PL emission spectra of BYSOP:yTb3+ samples excited at 247 and 377 nm and the corresponding variations of emission intensities (Figure S2), energy dispersive spectrum (EDS) of Ba2Y2(SiO4)(PO4)2 host and corresponding elemental analysis (Figure S3), and absorption spectrum of representative sample of BYSPO:0.14Ce3+,0.14Tb3+ (Figure S4) (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J.L.). *E-mail: [email protected] (H.L.). K


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Photoluminescence and Energy Transfer Properties with Y+SiO4

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