Luminescent properties and energy transfer mechanism from Tb3+ to Eu3+ in CaMoO4:Tb3+,Eu3+ phosphors

JOURNAL OF RARE EARTHS, Vol. 34, No. 3, Mar. 2016, P. 251

Luminescent properties and energy transfer mechanism from Tb3+ to Eu3+ in CaMoO4:Tb3+,Eu3+ phosphors XIONG Jianhui (熊健会), MENG Qingyu (孟庆裕)∗, SUN Wenjun (孙文军) (Key Laboratory for Photonic and Electronic Bandgap Materials, Ministry of Education, School of Physics and Electronic Engineering, Harbin Normal University, Harbin 150025, China) Received 4 August 2015; revised 18 December 2015

Abstract: A series of CaMoO4:Tb3+,Eu3+ phosphors were prepared by the method of precipitation. The structure and morphology of the phosphors were characterized by the X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM). The photoluminescence properties of the prepared products were researched, and the energy transfer from Tb3+ to Eu3+ in CaMoO4 phosphors was studied. By adjusting the doping concentration of Eu3+ ions in CaMoO4:Tb3+,Eu3+ phosphors, the emitting color of the phosphors could be easily tuned from green to red. With Tb3+ doped in the phosphors, the red luminescence of Eu3+ by near UV excitation was significantly enhanced. The energy transfer efficiency, rate and average distance between Tb3+ and Eu3+ in CaMoO4:5%Tb3+,x%Eu3+ (mole percent) phosphors (x=0.3–10) were calculated. It was found that the interaction type between Tb3+ and Eu3+ was electric dipole-dipole interaction in the phosphors. Keywords: Eu3+; Tb3+; luminescent properties; energy transfer; rare earths

In recent decades, people have already found the luminescence of rare earth ions in solids. Because rare earth elements have special electronic configuration, their 4f orbit have rich energy level structure that made rare earth luminescent materials have abundant emission spectra[1–8]. In recent years, rare earth polychromatic luminescence materials in the area of displays, lightings, lasers and photoelectric devices attract more and more people’s attention[9–14]. Molybdate material has excellent optical, electromagnetic properties and chemical stability, in the display, the areas of lighting, optical communication, and chemical catalysis has a wide range of applications, therefore, molybdate matrix has become one of the research emphasis for related researchers. Due to the effect of polarization of MoO42–, Eu3+ doped CaMoO4 possesses strong absorption corresponding to Eu3+ 4f-4f transition in near UV and blue light region[15,16], and it is generally considered as a good red phosphor[17–22]. Tb3+ is an excellent green light activator ion. The energy can transfer from Tb3+ to Eu3+ in CaMoO4 phosphors[23–25], and maybe it can improve the luminescence properties of Eu3+ doped CaMoO4 phosphors. The emission color of Eu3+, Tb3+ codoped CaMoO4 phosphors can change from green to red by doping different proportions of Eu3+ and Tb3+. It suggested that the kind of phosphors has the potential as materials for anti-counterfeiting technologies[26]. It is very useful to study the change of luminescence

color and intensity in Tb3+, Eu3+ codoped CaMoO4 phosphors and reveal the physical mechanism of energy transfer from Tb3+ to Eu3+ in the phosphors. Considering the above, we synthesized Tb3+, Eu3+ codoped CaMoO4 micro-crystals by precipitation method in this work. We observed the X-ray diffraction spectra and scanning electron microscopy images of the simples. The excitation spectra, emission spectra and the fluorescence decay curves of the samples were gauged. The luminescent properties and energy mechanism from Tb3+ to Eu3+ in CaMoO4:Tb3+,Eu3+ phosphors were studied.

1 Experimental Chemical precipitation method was used because this method has been widely used in the synthesis of mental oxide crystal powders and ceramic materials. Through a large number of experiments, we found a good reaction condition. The Eu(NO3)3, Tb(NO3)3 and Ca(NO3)3 were put into 20 mL deionized water and we adjusted the PH to 1 with nitric acid. The Na2MoO4 was dissolved in 40 mL deionized water and we adjusted the pH to 10 with ammonia water. The solution of Eu(NO3)3, Tb(NO3)3 and Ca(NO3)2 were dropped slowly into the Na2MoO4 solution and we mixed the solution the white precipitate arose. The turbid liquid was stirred 1 h to complete reaction after all the mixture solution was dropped. After re-

Foundation item: Project supported by the National Natural Science Foundation of China (51002041) and Natural Science Foundation of Heilongjiang Province (F201202) * Corresponding author: MENG Qingyu (E-mail: [email protected]; Tel.: +86-451-88060526) DOI: 10.1016/S1002-0721(16)60022-4

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action, the precipitate was washed three times with deionized water. Then, the precipitate was dried at 80 ºC for 3 h. The dried precipitate was ground and then calcined for 1 h in 800 ºC in a muffle furnace and the precipitate was ground again. In this work, fifteen samples were prepared, the doping concentrations (mole percent) are CaMoO4: x%Tb3+ (x=2, 3, 4, 5, 6, 7, 8) and CaMoO4:5%Tb3+, x%Eu3+ (x=0, 0.3, 0.5, 1, 3, 5, 7, 10) respectively. The phase purity of the products was examined by using X-ray diffraction (XRD) using a Neo-confucianism Japanese company D/max2600 diffractometer with Cu Kα radiation (λ=0.15406 nm) as X-ray source. The XRD data of 2θ° range from 10° to 70° were collected by using a scanning mode with a scanning step of 0.02° and a scanning rate of 2(°)/min. The shape, size, and morphology were examined by a field scanning electron microscope (FE-SEM) (Hitachi S4800). Photoluminescence (PL) emission, excitation spectra and fluorescent decay curves were recorded with an Edinburgh Instruments LFS920 fluorescence spectrometer equipped with a 450 W xenon lamp as an excitation source. All the measurements are under room temperature in every experiment.

Tb3+ doping concentration of 5% and Eu3+ doping concentration from 0% to 10%. The crystal lattice is bodycentered tetragonal (BCT), corresponding to JCPDS card #85-1267. From Fig. 1, all of the observed peaks satisfy the reflection conditions, confirming the formation of a single phase with no impurities. We can also see that with a small amount of Tb3+ ions and Eu3+ ions doped in CaMoO4 phosphors, the diffraction peaks position of CaMoO4 phosphors are not changed, which indicate that doping little rare earth ions nearly had no effect on the crystal structure of the samples. Figs. 2 (a–d) show the FE-SEM images of the

2 Results and discussion 2.1 Crystal structure and morphology of the prepared products Fig. 1 shows the XRD patterns of the samples with

Fig. 1 XRD patterns of the prepared products of CaMoO4:5% Tb3+,x%Eu3+ (x=0, 1, 5, 10)

Fig. 2 SEM images of Eu3+, Tb3+ doped CaMoO4 phosphors (a, b) 5%Tb3+,1%Eu3+; (c, d) 5%Tb3+,5%Eu3+

XIONG Jianhui et al., Luminescent properties and energy transfer mechanism from Tb3+ to Eu3+ in …

CaMoO4:Tb3+,Eu3+ phosphors with (a, b) 5%Tb3+, 1%Eu3+ and (c, d) 5% Tb3+, 5% Eu3+. It can be seen that the particle size is uniformly distributed and about 1 μm.

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Fig. 3 Emission spectra of CaMoO4:x%Tb3+ phosphors (x=2, 3, 4, 5, 6, 7, 8) under 486 nm excitation (the illustration is concentration quenching curve of Tb3+ 5D4→7F5 transition which is excited by 486 nm)

tion spectra located at 200–340 nm in the broadband absorption is due to the charge transfer band of the O2––Mo6+ and Tb3+ 4f-5d transition. The peaks positions are 360, 369 and 378 nm corresponding to the ground state 7F6 to the excited states 5D2, 5G6 and 5D3[27]. There is a strong absorption peak in the excitation of 486 nm, corresponding to the characteristic transition of Tb3+: 7 F6→5D4. Curve (2) shows the excitation spectra of CaMoO4:5%Tb3+,1%Eu3+ monitoring 545.5 nm emissions corresponding to the Tb3+ 5D4→7F5 transition that is similar with curve (1). Curve (3) shows the excitation spectra of CaMoO4:5%Tb3+,1%Eu3+ and monitoring wavelength is 616 nm from Eu3+: 5D0→7F2, the excitation spectra located at 200–340 nm in the broadband absorption is due to the charge transfer band of the O2––Eu3+ and O2––Mo6+ transition. We can find the peaks positions of 362 nm (Eu3+ 7F0→5D4), 381 nm (Eu3+ 7 F0→5L7), 395 nm (Eu3+ 7F0→5L6), 416 nm (Eu3+ 7 F0→5D3), 465 nm (Eu3+ 7F0→5D2), and 486 nm (Tb3+ 7 F6→5D4). Comparing curve (2) with curve (3), when we monitor the luminescence of Tb3+, we can not see any characteristic excitation from Eu3+. However, when monitoring the luminescence of Eu3+, we can find the peak position at 486 nm that comes from the characteristic excitation of Tb3+. These indicate that the energy transfer from Eu3+ to Tb3+ is invalid and the energy transfer from Tb3+ to Eu3+ is effective. Fig. 5 shows the emission spectra of the samples at the excitation wavelength 486 nm and 394.5 nm. The concentrations are 5%Tb3+ and 5%Tb3+,1%Eu3+. Curve (1) consisted of the Tb3+ 4f-4f transition lines located at 545 nm ( 5 D 4 → 7 F 5 ), 587 nm ( 5 D 4 → 7 F 4 ), and 621 nm (5D4→7F3)[27]. Curve (3) shows the Eu3+ 4f-4f transitions: 592 nm ( 5 D 0 → 7 F 1 ), 616 nm ( 5 D 0 → 7 F 2 ), 655 nm (5D0→7F3), and 702 nm (5D0→7F4). Curve (2) shows the Eu3+ 4f-4f transition and Tb3+ 4f-4f transition emission peaks. Through comparing curve (3) with curve (2), when we use the characteristic excite wavelength of Eu3+ (394.5 nm) to excite the CaMoO 4:5%Tb3+,1%Eu3+

Fig. 4 Excitation spectra of CaMoO4:5%Tb3+ (λem=545.5 nm) (1), CaMoO4:5%Tb3+,1%Eu3+ (λem=545.5 nm) (2) and CaMoO4:5%Tb3+,1%Eu3+ (λem=616 nm) (3) phosphors

Fig. 5 Emission spectra of CaMoO4:5%Tb3+ (λex=486 nm) (1), CaMoO4:5%Tb3+,1%Eu3+ (λex=486 nm) (2) and CaMoO4:5%Tb3+,1%Eu3+ (λex=394.5 nm) (3) phosphors

2.2 Luminescent properties of CaMoO4:Tb3+,Eu3+ Fig. 3 shows the emission spectra of CaMoO4:x%Tb3+ (x=2, 3, 4, 5, 6, 7, and 8) phosphors and the excitation wavelength is 486 nm. The illustration in Fig. 3 is concentration quenching curve of Tb3+ 5D4→7F5 transition green luminescence of CaMoO4 samples which is excited by 486 nm. With the increase of Tb3+ concentration, the luminescent intensity of 545.5 nm (Tb3+:5D4→7F5) first increases and then decreases. The best concentration of Tb3+ is 5%. When the Tb3+ concentration is more than 5%, the luminescent intensity remarkably decreases. This phenomenon is due to the energy transfer between Tb3+ ions leading to the concentration quenching. Therefore we chose the Tb3+ concentration of 5% in this work. Fig. 4 shows the excitation spectra of the CaMoO4 phosphors. Curve (1) shows the excitation spectra of CaMoO4:5%Tb3+ which monitors 545.5 nm emission corresponding to the Tb3+ 5D4→7F5 transition, the excita-

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phosphors, we can only see the emissions of Eu3+, but when we use the characteristic excitation wavelength of Tb3+ (486 nm) excited the phosphors, we can find both emissions of Tb3+ and Eu3+. So we further identify that the energy transfer from Eu3+ to Tb3+ is invalid and from Tb3+ to Eu3 is effective, this conclusion is consistent with the above. Fig. 6 shows the emission spectra of CaMoO4:5%Tb3+, x%Eu3+ (x=0, 0.3, 0.5, 1, 3, 5, 7 and 10) phosphors by 486 nm excitation. With the increase of Eu3+ concentration in the phosphors, the emission intensities of Tb3+ remarkably decrease and the emission intensity of Eu3+ increases, indicating that energy can transfer from Tb3+ to Eu3+ in CaMoO4 phosphors. From above observation it can be concluded that the emission colors of CaMoO4: Tb3+,Eu3+ phosphors can be tuned by changing the doping concentration of Eu3+. For the sake of further quantificational study the effect of Eu3+ concentration on luminescent colors, the CIE (Commission International del’Eclairage) chromaticity coordinates of the CaMoO4: 5%Tb3+,x%Eu3+ (x=0, 0.3, 0.5, 1, 3, 5, 7 and 10) phosphors were calculated by the emission spectra and are shown in Fig. 7. The right side of Fig. 7 shows the dark field real shot of CaMoO4 phosphors. On the basis of the data in Fig. 7, it can be seen that with the increase of Eu3+ concentration, the color of the luminescence can be adjusted from green to red. Fig. 8 shows the emission spectra of CaMoO4:5%Tb3+, x%Eu3+ (x=0, 0.3, 0.5, 1, 3, 5, 7 and 10) phosphors by 274 nm excitation. The law of the spectra is similar to Fig. 6. The CIE (Commission International del’Eclairage) chromaticity coordinates of the CaMoO 4:5%Tb 3+ , x%Eu3+ (x=0, 0.3, 0.5, 1, 3, 5, 7 and 10) phosphors were calculated by the emission spectra that excitation wavelength is 274 nm as revealed in Fig. 9. The right side of Fig. 9 shows the dark field real shot of CaMoO4 phosphors. On the basis of the data in Fig. 9, it can be seen that while the concentration of Eu3+ increased, the color of the luminescence can also be adjusted from green to red. Comparing Fig. 7 with Fig. 9, it can be clearly seen that x value of the chromaticity coordinates in Fig. 9 is

Fig. 6 Emission spectra of CaMoO4:5%Tb3+,x%Eu3+ phosphors (x=0, 0.3, 0.5, 1, 3, 5, 7, 10) under 486 nm excitation

JOURNAL OF RARE EARTHS, Vol. 34, No. 3, Mar. 2016

Fig. 7 CIE chromaticity coordinates and dark field real shot of CaMoO4:5%Tb3+,x% Eu3+ phosphors (x=0, 0.3, 0.5, 1, 3, 5, 7 and 10) (the excitation wavelength is 486 nm)

Fig. 8 Emission spectra of CaMoO4:5%Tb3+,x% Eu3+ phosphors (x=0, 0.3, 0.5, 1, 3, 5, 7, 10) under 274 nm excitation

Fig. 9 CIE chromaticity coordinates and dark field real shot of CaMoO4:5%Tb3+,x%Eu3+ phosphors (x=0, 0.3, 0.5, 1, 3, 5, 7 and 10) (the excitation wavelength is 274 nm)

XIONG Jianhui et al., Luminescent properties and energy transfer mechanism from Tb3+ to Eu3+ in …

larger than in Fig. 7 with the same Eu3+ concentration. In other words, the red emission component of the phosphors at 274 nm excitation is larger than the red emission component at 486 nm excitation. This is because that 274 nm can stimulate both Tb3+ and Eu3+, thus the luminescence of Eu3+ comes from the energy transfer from Tb3+ and its own emission. However, 486 nm can only stimulate the energy levels of Tb3+, and then the energy transfer from Tb3+ to Eu3+, the luminescence of Eu3+ can only come from the energy transfer of Tb3+. So the red luminescence component of the phosphors with 274 nm excitation is larger than that excited by 486 nm. Fig. 10 shows the excitation spectra (200–500 nm, λem=616 nm) of CaMoO4:10%Eu3+, CaMoO4:5%Tb3+, 10%Eu3+ and emission spectra (500–720 nm, λex=394.5 nm) of CaMoO4:10%Eu3+, CaMoO4:5%Tb3+,10%Eu3+ phosphors. It is easy to see from the figure that the luminescent intensity of curve (2) is slightly larger than that of curve (1). Doping Tb3+ in the phosphors changed the crystal field environment around Eu3+, and made the chaotic degree increase in Eu3+ crystal field, so the transition rate of Eu3+ also increased. Moreover, the excitation spectra of curve (2) have an absorption peak at about 486 nm; this can add the blue areas absorption of the phosphors. 2.3 Energy transfer mechanism from Tb3+ to Eu3+ In order to more clearly comprehend the energy transfer process between Tb3+ and Eu3+, Fig. 11. shows the fluorescent decay curves of CaMoO4:5%Tb3+,x% Eu3+ (x=0, 0.3, 0.5, 1, 3, 5, 7, 10) phosphors (λex=486 nm, λem= 545.5 nm). From the curves in Fig. 11, with the concentration of Eu3+ increasing, the lifetime of the luminescent centers gradually reduced. For further quantitatively studying the lifetime of the luminescence center as the change of Eu3+ doping concentration, the average fluorescent lifetime τs were calculated according to the following formula[28]

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Fig. 11 Fluorescent decay curves of CaMoO4:5%Tb3+,x%Eu3+ (x=0, 0.3, 0.5, 1, 3, 5, 7, 10) phosphors (λex=486 nm, λem=545.5 nm)

τs =

∫ tI (t )dt ∫ I (t )dt

(1)

where I(t) represents the fluorescent intensity at time t, and the average lifetimes of the luminescent centers are listed in Table 1 (λex=486 nm, λem=545.5 nm). We find the lifetime of the luminescent centers decrease along with Eu3+ doped in, which is because Eu3+ doping in the samples made the electrons have more opportunity to remove from higher energy levels to lower energy levels, and this can cause the time of the electrons to stop on the energy level decrease. The possible energy transfer channels attributed to the going out of 5D4 fluorescence of Tb3+ by Eu3+ are shown in Fig. 12. The reason of energy transfers from Tb3+ to Eu3+ very efficiently is that Eu3+ and Tb3+ have large emission spectra overlap[29]. To further quantitatively describe the energy transfer courses and comprehensively comprehend physical mechanism of the interaction between Tb3+ and Eu3+ in the CaMoO4 phosphors, we calculated the energy transfer efficiencies(ηET) based on the following equation[30,31]

η ET = 1 −

τs τs

(2)

0

In Eq. (2), τs is the lifetime of Tb3+ with the existence of Eu3+ and τs0 is the lifetime of Tb3+ in the samples Table 1 Average lifetime of Tb3+, energy transfer rate, efficiency, and average distance of CaMoO4:5%Tb3+, x%Eu3+ phosphors (x=0, 0.3, 0.5, 1, 3, 5, 7, 10) Eu3+ content/%

Fig. 10 Excitation spectra (200–500 nm, λ em=616 nm) of CaMoO4:10%Eu3+ (1), CaMoO4:5%Tb3+,10%Eu3+ (2) and emission spectra (500–720 nm, λex=394.5 nm) of CaMoO4:10%Eu3+ (1), CaMoO4:5%Tb3+,10%Eu3+ (2) phosphors

Tb3+ lifetime/ms

Ada/s–1

ηET/%

RTb-Eu/nm

0

0.64

–

–

–

0.3

0.523

349.55

18.33

1.411

0.5

0.472

556.14

26.25

1.394

1.0

0.419

824.13

34.58

1.354

3.0

0.357

1238.62

44.17

1.230

5.0

0.325

1514.42

49.08

1.142

7.0

0.261

2268.91

59.17

1.075

10.0

0.208

3245.19

67.50

0.998

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rent situation are all more than 0.5–0.7 nm. Therefore, the energy transfer through the exchange interaction is impossible. So the energy transfers from Tb3+ to Eu3+ in CaMoO4 phosphor by an electric multipole interaction. According to the Dexter’s energy-transfer formula of electric multipole interaction and Reisfeld approximation, the following relations are obtained[36–38]:

τs

α /3

(5) ∝C τs In the above equation, τs is the fluorescent lifetime of Tb3+ with single Tb3+ concentration in CaMoO4, and τs is the fluorescent lifetime of Tb3+ with the presence of Eu3+, C is Eu3+ concentration, α=6, 8, 10, represent electric dipole-dipole, electric dipole-quadrupole, and electric quadrupole-quadrupole interactions, respectively. The τs /τs~Cα/3 (C≥1%) are plotted as shown in Fig. 13, the best linear behavior can only be observed when α=6, this can illustrate that energy transfer from Tb3+ to Eu3+ by electric dipole-dipole interaction mechanism. 0

0

Fig. 12 A graphical model for the energy transfer from Tb3+ to Eu3+ in CaMoO4:Tb3+,Eu3+ phosphors

without Eu3+. In addition, the energy transfer rate (Ada) from Tb3+ to Eu3+ is also calculated by the following equation: 1 1 Ada = − (3)

τs

τs

0

Table 1 shows the calculation results of ηET and Ada, from the average lifetime, we found that with the increasing of Eu3+ ions concentration, both ηET and Ada are increased. In ordinary, the energy transfer can take place via exchange interaction or electric multipole interaction between luminescent centers of one host. For exchange interaction, the energy transfer rate is proportional to e–R, in the electric multistage interaction, energy transfer rate is proportional to R–α (R is the distance between the Tb3+ and Eu3+, α is electric multipole interaction index)[32]. Thus, the distance between Tb3+ and Eu3+ decreased greatly due to the increasing of Eu3+ concentration, and then the ηET and Ada would increase. Exchange interaction requires a lot of direct or indirect overlaps between the donor and receptor, leading to a simple electronic exchange. In other words, if R is less than the critical distance, energy exchange process of moving from the donor to the receptor will dominate. Otherwise, electric multipole interaction is more effective[33]. In general, the key of the donor and recipient should be shorter than the distance of 0.5–0.7 nm for the exchange interaction mechanism[34]. In this work, RTb–Eu can be confirmed by the relationship of the following equation[35]: 3V 1/ 3 (4) RTb − Eu ≈ 2[ ] 4 πX c N where Xc is the rare earth ions concentration, N is the positive ions in one unit cell, and V is the volume of the unit cell. For the CaMoO4, N=4, V=0.31186 nm3, therefore, the RTb–Eu of CaMoO4 5%Tb3+,x% Eu3+ were calculated to be 1.411, 1.394, 1.354, 1.230, 1.142, 1.075 and 0.998 nm with x=0.3, 0.5, 1, 3, 5, 7, 10 independently. The average distances of the donor-acceptor in the cur-

0

Fig. 13 Dependence of τs0/τs of Tb3+ on C6/3 (a), C8/3 (b) and C10/3 (c)

3 Conclusions We successfully synthesized Tb3+, Eu3+ codoped CaMoO4 micro-crystals by the method of precipitation. The structure and morphology of the samples were characterized by XRD and FE-SEM. The luminescent properties and the energy transfer mechanism from Tb3+ to Eu3+ in the phosphors were studied. With the increase of Eu3+ concentration, the luminescent colors could be changed from green to red easily because of different proportions of Tb3+ and Eu3+; the phosphors had potential application in the anti-counterfeiting technology. With Tb3+ doped in the phosphors, the red luminescence of Eu3+ by near UV excitation was significantly enhanced. The study also indicated that the energy transfer rate and energy transfer efficiency from Tb3+ to Eu3+ were increased with the increase of Eu3+ concentration, and the energy transfer type between Tb3+ and Eu3+ was electric dipole-dipole interaction.

XIONG Jianhui et al., Luminescent properties and energy transfer mechanism from Tb3+ to Eu3+ in …

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