Broadband Yellowish-Green Emitting Ba4Gd3

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Broadband Yellowish-Green Emitting Ba4Gd3Na3(PO4)6F2:Eu2+ Phosphor: Structure Refinement, Energy Transfer, and Thermal Stability Xiaopeng Fu,†,§ Wei Lü,*,† Mengmeng Jiao,†,‡ and Hongpeng You*,† †

State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Science, Changchun 130022, P. R. China ‡ University of the Chinese Academy of Science, Beijing 100049, P. R. China § College of Chemical Engineering, Northeast Dianli University, Jilin 132012, P. R. China

ABSTRACT: A series of Ba4Gd3Na3(PO4)6F2:Eu2+ phosphors with a broad emitting band have been synthesized by a traditional solid state reaction. The crystal structural and photoluminescence properties of Ba4Gd3Na3(PO4)6F2:Eu2+ are investigated. The different crystallographic sites of Eu2+ in Ba4Gd3Na3(PO4)6F2:Eu2+ phosphors have been verified by means of their photoluminescence (PL) properties and decay times. Energy transfer between Eu2+ ions, analyzed by excitation, emission, and PL decay behavior, has been indicated to be a dipole−dipole mechanism. Moreover, the luminescence quantum yield as well as the thermal stability of the Ba4Gd3Na3(PO4)6F2:Eu2+ phosphor have been investigated systematically. The as-prepared Ba4Gd3Na3(PO4)6F2:Eu2+ phosphor can act as a promising candidate for n-UV convertible white LEDs.

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The broad emission band of the Eu2+ ion benefits from the 4f−5d dipole allowed transition. The Eu2+ ion is known as one kind of excellent activator for phosphors, and the emission band is tunable in the entire visible range of electronic transitions.24−28 For the past few years, the Eu2+ ion as an activator mixed with many hosts has been studied, such as Sr 2 SiO 4 :Eu 2+ , 2 9 Ba 5 SiO 4 Cl 6 :Eu 2+ , 30 Li 2 SrSiO 4 :Eu 2+ , 3 1 Ca 2 PO 4 Cl:Eu 2+ , 32 Ca 2 Al 3 O 6 F:Eu 2+ , 33 K 2 Al 2 B 2 O 7 :Eu 2+ , 34 Sr9Mg1.5(PO4)7:Eu2+,35 Sr3Ce(PO4)3:Eu2+,36 Sr5(PO4)3‑x(BO3)xCl:Eu2+,37 etc. The fluorapatite structure type has been found and determined by Náray-Szábo in 1930 at first time, and the apatite-type framework is Ca10(PO4)6F2. The apatite-type structure belongs to the hexagonal symmetrical system. These compounds are generally expressed as A10(XO4)6Z2, where A is

INTRODUCTION Energy-exhausting, white light diodes as the fourth generation of lighting source have been a hot spot for global researchers. These have excellent physical and chemical characteristics, such as being environmentally friendly and having a long lifetime and high efficiency.1−5 The dominating method for true wLEDs is the combination of blue or n-UV chips with one or more phosphors.6−10 Currently, the most widely used commercial white LED lamps consist of blue and/or near UV LED chips and Y3Al5O12:Ce3+ (YAG:Ce)-based yellow phosphors in recent years. However, the only fly in the ointment, their unsatisfactory performance (low CCR and high CCT), restricts its further expansion of the application. Another way to obtain white light is the combination of n-UV chips with triphosphors; this can effectively overcome the problems aforementioned.11−23 In view of the excellent characteristics of the CRI and easily adjusted emission color properties it involves, it is important to obtain highly efficient n-UV-excited phosphor. © XXXX American Chemical Society

Received: March 15, 2016

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DOI: 10.1021/acs.inorgchem.6b00648 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry cation (Ca2+, Sr2+, Ba2+, La3+, Y3+, etc.), X is cation (P5+, Si4+, etc.), and Z is anion (F−, Cl−, etc.).38−42 Due to its excellent thermal and hydrolytic stability, and other remarkable chemical properties, single-component multichromatic phosphors with this structure have been prepared and investigated, such as La 9 . 3 3 (SiO 4 ) 6 O 2 :Ce 3 + ,Eu 2 + ,Mn 2 + , 4 3 Ca 6 Y 2 Na 2 (PO 4 ) 6 F2:Eu2+,Mn2+,44 Mg2Y8(SiO4)6O2:Ce3+,Mn2+,Tb3+,45 Sr3GdNa(PO4)3F:Eu2+,Mn2+,46 and so forth. As we have seen, the phosphor of Eu2+ doped Ba4Gd3Na3(PO4)6F2 has not yet been reported. In this paper, we present a fluorapatite structure type crystal, Ba4Gd3Na3(PO4)6F2:Eu2+; the crystal structure and photoluminescent properties have been studied further. With excitation by UV light at 365 nm, the PL spectra of the Ba4Gd3Na3(PO4)6F2:Eu2+ phosphor shows an intense and board band from 400 to 650 nm, which almost steps over the spectrum of visible light. Thus, it is believed that the Ba4Gd3Na3(PO4)6F2:Eu2+ can be used for UV- or n-UV-based white LEDs.

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spectra agree well with the reference structure of Ba4Nd3Na3(PO4)6F2 (JCPDS 71-1318),48 except for the overall slight peak shift to higher angles due to Gd (1.61 Å) substitution on Nd (1.64 Å) sites. No other diffraction peaks were found, so it proved the pure phase of phosphor and successful incorporation of Eu2+. To further research the crystal structure of the samples, Rietveld structure refinement of BGNPF is performed using the GSAS program. Figure 2 shows the experimental and calculated

EXPERIMENTAL SECTION

Synthesis. A series of Ba4‑xGd3Na3(PO4)6F2:xEu2+ (BGNPF) phosphors have been synthesized by a high temperature solid state reaction. All raw materials are analytical-grade pure. A stoichiometric mixture with the excess amount of BaF2 is ground in an agate mortar. After fully grinding, the mixture is placed in a crucible and then sintered at 1075 °C for 6 h in a reducing atmosphere (10% H2 + 90% N2). Characterization. The measurements of PLE and PL spectra are performed using a Hitachi F-7000 spectrometer equipped with a 150 W xenon lamp under a working voltage of 700 V. The structures of samples are identified by powder X-ray diffraction (XRD) analysis (Bruker AXS D8), with Cu Kα radiation (λ = 1.5418 Å) operating at 40 kV and 40 mA. Crystal structure refinement employed the Rietveld method, as implemented in the General Structure Analysis System (GSAS) program.47 The luminescence decay curve is obtained from a Lecroy Wave Runner 6100 digital oscilloscope (1 GHz) using a tunable laser (pulse width = 4 ns, gate = 50 ns) as the excitation source (Continuum Sunlite OPO).

Figure 2. XRD profiles for the Rietveld refinement of BGNPF:0.04Eu2+: experimental results (×), calculated results (|), and their difference (bottom). The green solid lines represent the Bragg reflection positions.

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RESULTS AND DISCUSSION To verify the phase, all samples were characterized by XRD. Figure 1 displays the XRD patterns of BGNPF:xEu2+. All of the

results and the difference results of XRD profiles for the Rietveld refinement of BGNPF:0.02Eu2+, respectively. The fluorophosphates BGNPF are isostructural to the apatite-type Ba4Nd3Na3(PO4)6F2 compounds, with space group P3̅ and cell parameters of a = b = 9.738 Å, c = 7.210 Å, V = 592.144 Å3. Their crystallographic data and selected bond lengths are shown in Table 1. There is one kind of cation site named M (column cation sites) in the Ba4Gd3Na3(PO4)6F2 host. The M cation, which contains a mixture of 2/3Ba, 1/6Gd, and 1/6Na, is coordinated by nine atoms (seven oxygen atoms and two fluoride). The M−O bond lengths range from 2.5 to 3.0 Å, resulting in an irregular polyhedron. The refinement also shows that the structure of the Ba4Gd3Na3(PO4)6F2 contains another Gd site ion coordinated by nine oxygen atoms with average bond lengths of 2.58 Å.48 When Eu2+ is doped in the crystal structure of BGNPF, in view of the effective ionic radii and charge balance, we suggest that Eu2+ may prefer to occupy Ba2+ sites. However, the ionic radius of ionic Eu2+ (r = 1.30 Å, CN = 9) is not only similar to that of Ba2+ (r = 1.47 Å, CN = 9) but also close to that of Gd3+ (r = 1.107 Å, CN = 9). Therefore, the Eu2+ may also be expected to occupy the Gd3+ sites. This

Figure 1. XRD patterns of the as-synthesized BGNPF and BGNPF:xEu2+. B


Article

Inorganic Chemistry Table 1. Crystallographic Data of BGNPF and Selected Bond Lengths crystallographic data of BGNPF radiation type/Å 2θ range/deg T/K symmetry space group a/Å b/Å c/Å α/deg β/deg γ/deg V/Å Rp/% Rwp/% χ2

1.5418 10−100 295 trigonal P3̅ 9.738 9.738 7.210 90 90 120 592.144 5.45 7.6 3.845

selected bond lengths (Å) M−O(1) M−O(2) M−O(3) M−O(3′) M−O(4) M−O(4′) M−O(4″) M−F(1) M−F(2) Gd−O(l, 1′,1″) Gd−O(2,2′,2″) Gd−O(3,3′,3″) Na−O(1,l′,1″) Na−O(2, 2′,2″)

2.651(7) 3.035(6) 2.632(6) 2.923(7) 2.576(6) 2.713(7) 2.933(7) 2.786(22) 2.934(4) 2.533(6) 2.460(7) 2.574(6) 2.482(7) 2.566(8)

P−O(1) P−O(2) P−O(3) P−O(4)

1.528(5) 1.533(8) 1.543(7) 1.529(7)

Eu2+ ions. The emission spectrum can be deconvoluted into two Gaussian components, including a band with its peak at 470 nm and a band with its peak at 550 nm. These two bands were named as Eu2+(I) and Eu2+(II), respectively. In addition, the shapes of PLE spectra are different when the excitation wavelength is set as 470 and 550 nm. Accordingly, we speculate that there are two types of Eu2+ luminescent centers in this system. The crystal structure of BGNPF supplies two different sites available for the cations, i.e., 9-fold coordinated Ba2+/ Gd3+site (7 oxygens and 2 fluorines) and 9-fold coordinated Gd3+ site (9 oxygens). Ba2+ and Gd3+ (7 oxygens and 2 fluorines) ions occupy the M sites with a ratio of 4:1. The numbers of Ba2+ ions are much more than Gd3+ ions at M sites. Accounting for ion valence, we therefore presume that Eu2+ ions are expected to occupy the M sites of Ba2+ preferably. Thus, considering the average bond distance for Ba−O (∼2.8 Å) is dramatically longer than that for Gd−O (∼2.58 Å, with nine oxygens), it is surmised that the crystal field effects for the Ba2+ site are weaker than that for the Gd3+ (nine oxygens) site. Because of this, we believe that the blue emission and yellow emission bands are assigned to Eu2+(I) occupying Ba2+ with weak crystal field and Eu2+(II) occupying Gd3+ (nine oxygens) with strong crystal field, respecitively. The decay curves of the two emissions of BGNPF:0.04Eu2+ under pulse laser excitation at 320 nm were measured. As shown in Figure 3b, we know that Eu2+ ions have two kinds of decay curves, and each one is fitted well with a single exponential function. The lifetimes of 0.46 and 1.28 μs correspond to the shorter and the longer emission, respectively, which further proves that Eu2+ ions have two kinds of emission centers. In order to investigate the effect of doping concentration on luminescence properites, a series of BGNPF:xEu2+ (x = 0.005, 0.01, 0.02, 0.04, 0.08, 0.12, 0.16, 0.20) had been synthesized. Figure 4 depicts the photoluminescence of BGNPF:xEu2+ phosphors. All of the PL spectra of BGNPF:xEu2+ are composed of two bands peaking at 470 and 550 nm under the excitation of 350 nm. Figure 5 illustrates the intensity of PL spectra of BGNPF:xEu2+ with different doping contents. As we can see, the fluorescent intensity is increasing with the increase of the Eu2+ ion concentration, and it achieves the maximum of luminous intensity when x is 0.04. Beyond this, the Eu2+ intensity decreases gradually. Blasse indicated that the critical distance (Rc) between Eu2+ ions can be estimated using the following equation:49

deduction is actually observed in the PL spectra of the samples presented in the following section. As an activator, Eu2+ ion has been single-doped in the host. Figure 3a displays the PL and PLE spectra of the

Figure 3. (a) PL and PLE spectra of the BGNPF:0.04Eu2+ phosphor. (b) Experimental data and single exponential fitting curves of the BGNPF:0.04Eu2+ under pulse laser excitation at 355 nm.

BGNPF:0.04Eu2+ phosphor. The excitation wavelengths are fixed at 481, 550, and 600 nm, respectively. It can be observed that the PLE shows a broad band from 230 to 430 nm due to the 4f7−4f65d1 transition of Eu2+ ions. Upon excitation at 350 nm, the emission spectrum of BGNPF:0.04Eu2+ exhibits a broad asymmetric emission band from 400 to 650 nm, with its peak at 481 nm and a shoulder around 550 nm, and this is assigned to the electric dipole allowed 4f65d1−4f7 transition of C


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Inorganic Chemistry

θ is 6, 8, and 10, representing electric dipole−dipole, dipole− quadrupole, and quadrupole−quadrupole interactions, respectively. The relationships of log I/x versus log x are shown in the inset to Figure 5. In this plot, it shows the slope of the fitting line is 1.57; the value of θ is calculated to be to 4.71, which means that the dominant concentration quenching mechanisms between Eu2+ ions occur via dipole−dipole interactions. To get a better understanding on the energy transfer process, the PL decay curves of the Eu2+(I) at 450 with different Eu2+ concentrations are presented in Figure 6. We can found that

Figure 4. Series of PL spectra of the BGNPF:xEu2+ (x = 0.005, 0.01, 0.02, 0.04, 0.08, 0.12, 0.16, 0.20) phosphor.

Figure 6. Typical luminescence decay curves of Eu2+(I) with different Eu2+ concentrations.

the corresponding luminescent decay times reduce quickly and trend to be nonexponential function with increasing doped Eu2+ content x. The values of decay times for Eu2+(I) (τ1) and Eu2+(II) (τ2) are summarized in Figure 7. The lifetimes of

Figure 5. Dependence of the integrating intensity on the concentration of Eu2+ ions. The inset picture is the linear fitting of log(x) versus log(I/x) in the BGNPF:xEu2+ samples.

⎡ 3V ⎤1/3 R C ≈ 2⎢ ⎥ ⎣ 4πxcN ⎦

(1)

Here N is the number of host cations for Eu2+ ion substitution in per unit cell, V represents the volume of the unit cell, and xc is the critical concentration of doped ions. For the BGNPF host, V = 592.144 Å3, N = 7, and xc = 0.04; therefore, the critical distance RC is calculated to be 15.9 Å. It is known that the energy transfer mechanism includes radiation reabsorption, exchange coupling, or electric multipolar interactions. On the basis of the Dexter theory, the dominant energy transfer mechanism in Eu2+-activated BGNPF is electric multipolar interactions. According to the report of Van Uitert, the interaction type between sensitizers can be calculated by following the equation50,51 1 1 = x 1 + β(x)θ /3

Figure 7. Calculated average lifetimes of Eu2+(I) and Eu2+(II).

Eu2+(I) were calculated as 426, 393, 367, 343, 335, 315, 299, and 278 ns. The decay lifetime of Eu2+(I) decreases, which strongly supports the characteristics of energy transfer from Eu2+(I) to Eu2+(II). Comparatively, the lifetime of Eu2+(II) remains essentially unchanged. As we know, under steady state excitation, the rate equations describing the Eu2+(I) and Eu2+(II) can be written by eqs 3 and 4, respectively.

(2)

where I is the emission intensity, x is the activator concentration, k and β are constants for the same host crystal under the same excitation conditions. The type of nonradiative energy transfer can be indicated by the constant θ. The value of

f1 = n1/τ1

(3)

f2 + Wn1 = n2 /τ2

(4)

Here, f1 and f 2 correspond to the absorbing populations of Eu2+(I) and Eu2+(II), and n1 and n2 are the populations of D


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Inorganic Chemistry Eu2+(I) and Eu2+(II) ions, respectively; the energy transfer rates W can be given by W = 1/τ1 − 1/τ0. On the basis of energy transfer, the dependences of the emission intensity ratios of Eu2+(II) to Eu2+(I) on Eu2+ concentration can be strongly influenced by the fluorescence lifetimes of Eu2+(τ1) and Eu2+(τ2). Using eqs 3 and 4, the x dependence of the yellow/blue(I2/I1) ratio obtained from PL spectra on Eu2+ concentrations be expressed as52,53 ⎞ ⎞ n2γ2 γ ⎛ τ2f γ ⎛τ ⎛ f I2 τ ⎞ ∝ = 2 ⎜⎜ 2 + τ2W ⎟⎟ = 2 ⎜⎜ 2 ⎜⎜ 2 + 1⎟⎟ − 2 ⎟⎟ I1 n1γ1 γ1 ⎝ τ1f1 γ1 ⎝ τ1 ⎝ f1 τ0 ⎠ ⎠ ⎠ (5)

where γ1 and γ2 are radiative rates of Eu (I) and Eu (II), respectively. There values are independent of Eu2+ concentrations. τ1 and τ2 have been presented in Figure 7. In eq 5, the γ2/γ1 and f 2/f1 are unchanged for different excitation wavelength. Figure 8 shows the intensity ratios I2/I1 versus 2+

2+

Figure 9. (a) Temperature dependence of PL intensity of Eu2+ in BGNPF:0.1Eu2+ under excitation at 365 nm. (b) Fitted activation energy for thermal quenching of BGNPF:0.1Eu2+ using the Arrhenius equation.

theory of thermal quenching, the activation energy ΔE can be calculated by the following expression Figure 8. Intensity ratios I2/I1 vs τ2/τ1 at various Eu2+ concentrations.

I (T ) =

I(0) 1 + A exp( −ΔE /kBT )

(6)

in which I(0) is the initial emission intensity of the phosphor at room temperature, and I(T) is the emission intensity at different temperatures. A is a constant, T is the temperature, and k is the Boltzmann constant. ΔE is the activation energy for thermal quenching. The experimental data are well-fitted by eq 6, as shown in Figure 9b. Here, activation energy of Eu2+ in BGNPF is 0.18 eV. Figure 10 demonstrates the Commission International de L’Eclairage (CIE) chromaticity diagram for several typical BGNPF:xEu2+ samples, together with their corresponding photographs. The emission color of the phosphors shows a regular change, and the corresponding CIE coordinates of BGNPF:xEu2+ change from (0.238, 0.337) to (0.266, 0.401), which is due to the different concentrations of Eu2+ ions occupied in the sites of Ba2+ and Gd3+. The absorption spectra of BGNPF:Eu2+ match well with an n-UV chip, which means that the BGNPF:Eu2+ phosphor may be a potential material suitable for the demand of different color chromaticity.

τ2/τ1 at various Eu2+ concentrations. For comparison, the intensity ratios I2I1 obtained directly from the emission spectra are in good agreement with τ2/τ1. This result demonstrates that the energy transfer from Eu2+(I) to Eu2+(II) plays the main role in the I2/I1. One may also observe that the dots deviate from straightness for x = 0.02 and 0.04; the deviation is attributed to variation of f 2/f1 at these positions. In other words, when Eu2+ concentration is 0.04 mol, the distribution of Eu2+ in different sites may change significantly. The absolute quantum yield (QY) is a valuable and key parameter for LED actual application. Thus, we measured the absolute QY of all the samples. The absolute QY of BGNPF:xEu2+ (x = 0.005, 0.01, 0.02, 0.04, 0.08, 0.12, 0.16, 0.20) phosphors was determined to be 5.6.%, 15.1%, 27.5%, 48.9%, 52.3%, 56.7%, 41.9%, 32.5% at 365 nm excitation. The results reveal that BGNPF is a good matrix for rare earth ion doping. In the white LED application, the thermal quenching of phosphor luminescence is critical. Figure 9 shows the thermal stability of the BGNPF:0.1Eu2+ sample. There is a 50% loss of emission intensity after the temperature rises to 150 °C. Besides, the slight blue shift of the emission band can be observed with increasing temperature. This phenomenon can be ascribed to the thermally active phonon-assisted tunneling from the excited states of the low energy emission band to the excited states of the high energy emission band in the configuration coordinate diagram.54 According to the classical

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CONCLUSION In this study, a series of Eu2+-activated Ba4Gd3Na3(PO4)6F2 phosphors has been prepared. The Ba4Gd3Na3(PO4)6F2:Eu2+ crystallized in a hexagonal unit cell with cell parameters a = b = 9.738 Å, c = 7.210 Å, V = 592.144 Å3. The obtained Ba4Gd3Na3(PO4)6F2:xEu2+ has a broad excitation spectrum ranging from 230 to 430 nm, which can match perfectly with the commercial n-UV LED chips. The PL spectra exhibit two E


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Inorganic Chemistry

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Figure 10. CIE chromaticity diagram of the BGNPF:xEu2+ (x = 0.005, 0.04, 0.20) phosphor. The inset shows the image of the BGNPF:xEu2+ phosphor under 365 nm UV lamps.

kinds of emission peaks, 470 and 550 nm, originating from the presence of two different potassium sites in the Ba4Gd3Na3(PO4)6F2 host, respectively. The optimal doping concentration of Eu2+ is 4 mol %. In addition, we analyzed the ratio of two different Eu2+ emissions via theoretical calculation. The temperature dependent PL intensity of Ba4Gd3Na3(PO4)6F2:0.04Eu2+ were measured, and the activation energy was calculated to be 0.18 eV. Finally, the corresponding CIE coordinates of Ba4Gd3Na3(PO4)6F2:xEu2+ can change from (0.238, 0.337) to (0.266, 0.401).

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Fax: +86-431-85698041. Phone: +86-431-85262798. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by the National Natural Science Foundation of China (Grants 11304309 and 51472236), the National Basic Research Program of China (973 Program, Grant 2014CB643803), and the Fund for Creative Research Groups (Grant 21521092).

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