Adjustable emission and energy transfer process in BaGd2O4:Bi3+,Eu3+ phosphors

Author’s Accepted Manuscript Adjustable emission and energy transfer process in BaGd2O4:Bi3+,Eu3+ phosphors Hui Wang, XueYan Chen, LiMing Teng, DengKe Xu, WeiPing Chen, RongFei Wei, FangFang Hu, XinYuan Sun, Hai Guo www.elsevier.com/locate/jlumin

PII: DOI: Reference:

S0022-2313(18)30708-7 https://doi.org/10.1016/j.jlumin.2018.10.050 LUMIN15998

To appear in: Journal of Luminescence Received date: 20 April 2018 Revised date: 30 August 2018 Accepted date: 8 October 2018 Cite this article as: Hui Wang, XueYan Chen, LiMing Teng, DengKe Xu, WeiPing Chen, RongFei Wei, FangFang Hu, XinYuan Sun and Hai Guo, Adjustable emission and energy transfer process in BaGd2O4:Bi3+,Eu3+ p h o s p h o r s , Journal of Luminescence, https://doi.org/10.1016/j.jlumin.2018.10.050 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Adjustable emission and energy transfer process in BaGd2O4:Bi3+,Eu3+ phosphors Hui Wang a, XueYan Chen a, LiMing Teng a, DengKe Xu a, WeiPing Chen a, RongFei Wei a, FangFang Hu a, XinYuan Sun b, Hai Guo a,*

a. Department of Physics, Zhejiang Normal University, Jinhua, Zhejiang, 321004, China b. Department of Physics, Jinggangshan University, Ji’an 343009, P. R. China

*

corresponding author, E-mail: [email protected]

Abstract: A batch of Bi3+/Eu3+ single and co-doped BaGd2O4 phosphors were elaborated by traditional high temperature solid-state method. Their structural and photoluminescent properties were methodically studied by X-ray diffraction (XRD), scanning electron microscope (SEM), excitation spectra, emission spectra, decay curves as well as temperature dependent emission spectra. Under ultraviolet (UV) excitation, via a high-efficiency energy transfer process, the prepared Bi3+/Eu3+ co-doped BaGd2O4 phosphors reveal both blue wide emission from Bi3+ ions and characteristic red sharp emission from Eu3+ ions. The energy transfer mechanism, energy transfer efficiency and luminescence chromaticity coordinates were procured. Our results indicate that BaGd2O4:Bi3+,yEu3+ phosphors may act as promising candidate to boost the photosynthesis of plant.

Keywords: rare earth ions; phosphors; energy transfer; tunable luminescence.

1. Introduction In the past decades, rare-earth (RE) ions doped luminescent materials have been extensively investigated

[1-12]

because of their application in laser materials, plant

photosynthesis phosphors, white lighting emitting diodes (w-LEDs) conversion phosphors, etc. The electronic transitions of RE3+ ions come from 4f-4f transitions. Owing to the shield of 4f electrons by outer 5s25p6 electrons, RE3+ ions always possess sharp spectral lines [5]. For example, Eu3+ ion is an important red emitting activator ion, whose emissions come from 5D0-7FJ (J = 0-6) transitions [13, 14]. More recently, blue, green and red phosphors excited by near-ultraviolet (n-UV) source have attracted special attention n-UV LED chip to produce w-LEDs

[15, 16]

. These phosphors can be combined with

[17-29]

. Artificial lighting for plant cultivation is

an important factor in plant cost and nutritional quality of greenhouse vegetables. Chlorophylls predominantly absorbed blue and red light owing to the light sensitivity of plant

[30-32]

. Therefore, blue and red phosphors can be used to improve the

photosynthesis of plant

[33]

. However, because of the parity forbidden feature of f-f

inner transitions, the absorption of RE3+ ions from f-f transitions is very weak at n-UV region

[5, 20]

. Hence, in order to strengthen the excitation efficiency and promote the

luminescence of Eu3+ ions, several methods have been used, including charge transfer absorption, host absorption, and energy transfer (ET) process from sensitizer ions [5, 24]. On the other hand, Bi3+ ion is one of the most entirely researched main-group ions, whose electronic configuration is [Xe]4f145d106s2. Its transition comes from the parity allowed 6s6p-6s2 transition nm) in different hosts

[5]

. It can emit blue to green and even red bands (400-700

[34]

, which depends strongly on the type of covalence,

coordination number and site symmetry. As a result, Bi3+ ion can serve as not only an activator but also a sensitizer in luminescent materials [15, 16, 35, 36]. In this paper, BaGd2O4 is selected as host matrix. It is isostructural to CaFe2O4, and belongs to the family of ARE2O4 (where A = Ca, Sr, Ba) type binary compounds. In ARE2O4 compounds, there are one A site and two RE sites. Both RE sites occupy in Cs point symmetry without inversion center, which are very suitable for the doping of Eu3+

[7, 37-40]

. Herein, ET process from Bi3+ to Eu3+ in BaGd2O4 phosphors were researched in

details. Blue and red lights can be obtained by tuning Eu3+ and Bi3+ content.

2. Experimental procedures Phosphors with composition of BaGd2-xO4:xBi3+ (x = 0, 0.008, 0.016, 0.032, 0.06, 0.08, 0.1), BaGd1.8O4:0.2Eu3+ and BaGd1.92-yO4:0.08Bi3+,yEu3+ (y = 0, 0.01, 0.02, 0.08, 0.12, 0.2, 0.3) were elaborated by a solid-state reaction method. The starting materials are BaCO3 (A.R.), Bi2O3 (99.99%), Gd2O3 (99.99%) and Eu2O3 (99.99%). An excess of 2wt% H3BO3 (A.R.) was used as flux. Stoichiometric raw materials were blended and thoroughly grounded for 0.5 h in agate mortar. The powder mixtures were moved into crucibles and then sintered at 900 °C for 4 h in air. After grinding repeatedly, they were sintered at 1300 °C for 10 h in air and the final products were gained after cooling down to the room temperature. X-ray diffraction (XRD) patterns were characterized by a Rigaku MiniFlex/600 XRD apparatus (Tokyo, Japan) worked at 45 kV and 15 mA with CuKa radiation (λ = 0.154056 nm). The morphologies were examined by scanning electron microscope (Phenom ProX desktop SEM). The excitation and emission spectra were obtained by an Edinburgh FS5 spectrofluorometer. A continuous wave 150 W Xe lamp was used as excitation source and a TCB1402C temperature controller (China) was used to measure the temperature dependent luminescent spectra. Decay curves were performed by an Edinburgh FLS920 spectrofluorometer excited by a nanosecond flashlamp (nF900). All measurements were performed at room temperature except temperature-dependent spectra.

3. Result and discussion Fig. 1(a) gives the typical XRD patterns of BaGd2O4:Bi3+,Eu3+ phosphors. All the scanning peaks could be well indexed with standard data of orthorhombic BaGd2O4 (JCPDS No. 82-2320), which indicates the formation of pure orthorhombic BaGd2O4 (space group Pnam (62)) CN = 6)

[2, 15]

[7]

. Both Bi3+ (r = 1.03 Å, CN = 6)

[2, 15]

and Eu3+ (r = 0.947Å,

ions are suggested to occupy the sites of Gd3+ (r = 0.938 Å, CN = 6)

[37]

ions due to their similar radii and valence states. No obvious impurity phase is detected, indicating Bi3+ and Eu3+ dopants never cause any significant change [1, 16]. The

morphologies

of

BaGd2O4:0.08Bi3+ and

BaGd2O4:0.08Bi3+,0.2Eu3+

phosphors are characterized by SEM images, as shown in Fig. 1(b). The phosphors demonstrate the homogenous dispersion with irregular morphologies and smooth facets. The sizes of BaGd2O4:xBi3+ particles are in the range of sub-micrometers to several micrometers. Excitation spectra (λem = 432 nm) and emission spectra (λex = 337 nm) of BaGd2-xO4:xBi3+ phosphors are displayed in Fig. 2(a) and (b), respectively. BaGd2-xO4:xBi3+ samples show two main excitation bands ranging from 230 to 370 nm peaked at 256 nm and 337 nm, respectively. The ideal excitation band centered at 337 nm could be attributed to the spin-allowed 1S0-3P1 transition of Bi3+ weaker excitation band comes from 1S0-1P1 transition of Bi3+

[15, 36, 41, 42]

. The

[15, 36]

. Emission spectra

display broadband blue emission from 360 to 569 nm peaked at 432 nm, which could be contributed to 3P1-1S0 transition of Bi3+

[15, 36, 43]

. From Fig. 2, the emission and

excitation intensities of Bi3+ increase with Bi3+ content first, and reach the optimal content at x = 0.08, and then decrease with the raising of Bi3+ content as a result of concentration quenching effect. Excitation spectra (λem = 611 nm) of BaGd2O4:0.2Eu3+ and emission spectra (λex = 337 nm) of BaGd2O4:0.08Bi3+ are depicted in Fig. 3(a). In excitation spectra, there are a broadband ranging from 228 to 338 nm peaked at 280 nm, which could be attributed to Eu3+-O2- charge transfer transition. Several weak excitation peaks at about 272, 313, 363, 394, 465, 527 nm can be observed. The former two peaks originate from 8S7/2-6I9/2, 6

P3/2 transitions of Gd3+ and the others originate from 5F0-5D4, 5L6, 5D2 and 5D1

transitions of Eu3+

[1, 20, 37]

. As displayed in Fig. 3(a), BaGd2O4:0.08Bi3+ exhibits

broadband emission at 360-569 nm, while BaGd2O4:0.2Eu3+ shows absorption ranging from 360 to 550 nm. It forecasts that there is big spectral overlap between excitation of Eu3+ and emission of Bi3+, foreboding efficient ET process would be occurred from Bi3+ to Eu3+.

Fig. 3(b) presents the normalized excitation spectra of BaGd2O4:0.2Eu3+ and BaGd2O4:0.08Bi3+,0.2Eu3+ (λem = 611 nm), and BaGd2O4:0.08Bi3+ (λem = 432 nm). It is clear there exists a new excitation band from 305 to 370 nm in Bi3+,Eu3+ co-doped sample, which is similar with excitation band of Bi3+ doped sample. Such phenomenon indicates the existence of ET process from Bi3+ to Eu3+. Fig. 4(a) depicts the emission spectra (λex = 337 nm) of BaGd2O4:0.08Bi3+,yEu3+ samples. 337 nm light excitation, as the first-rank excitation for Bi3+ and not for Eu3+, the emission spectra of BaGd2O4:0.08Bi3+,yEu3+ samples exhibit both Bi3+ emission at blue band and Eu3+ emission peaks at red range. The emission intensities of Bi3+ decline with the raising of Bi3+ content. With the raising of Eu3+ content, the emission intensities of Bi3+ decline, while the emission intensities of Eu3+ raise firstly, reach maximum for y = 0.2, and then decline as a result of concentration quenching process. Fig. 4(a) points out that the quenching content of Eu3+ in BaGd2O4 is y = 0.2. It is clear in Fig. 4(b) that there is a diverse of emission intensities from Bi3+ and Eu3+. Fig. 4(a) and (b) state undoubtedly the high emission output of Eu3+ practically is from the ET process from Bi3+ to Eu3+. In addition, the ET efficiency from Bi3+ to Eu3+ could be counted by formula [1, 15, 24]

, T  1  I S I S 0

(1)

where IS and IS0 are the integrated luminescence intensity of Bi3+ ions with and without Eu3+ co-doping, respectively. Excited by 337 nm light, the ET efficiencies, as plotted in Fig. 4(b), are about 16.4, 37.9, 51.1, 70.2, 84.3 and 93.3% for BaGd2O4:0.08Bi3+,yEu3+ samples (y = 0.01, 0.02, 0.08, 0.12, 0.2, 0.3), respectively, which forecasting that ET process is very efficient. To provide a persuasive evidence for ET from Bi3+ to Eu3+, the decay curves of Bi3+ emission in BaGd2O4:0.08Bi3+,yEu3+ phosphors were systemically investigated. Fig. 5 presents the decay curves of 432 nm emission of Bi3+ (λex = 337 nm) in BaGd2O4:0.08Bi3+,yEu3+ phosphors. It is clear that all curves are non-exponential.

Therefore the decay processes are represented by mean lifetime  , which could be calculated by the following formula [5],

   tI  t  dt

 I t  dt

(2)

where I(t) represents the emission intensity at time t. The corresponding mean lifetimes of Bi3+ emission calculated are 422, 407, 352, 302, 255, 188, and 161 ns for BaGd2O4:0.08Bi3+,yEu3+ (y = 0, 0.01, 0.02, 0.08, 0.12, 0.2, 0.3) samples, respectively. The raising of Eu3+ doping content results in faster decay, which could be ascribed to ET process from Bi3+ to neighboring Eu3+. To investigate the ET type from Bi3+ to Eu3+, the critical distance (Rc) between Bi3+ and Eu3+ in BaGd2O4 phosphors are estimated by Blasse’s formula [1], 1

 3V  3 Rc  2    4 X c N 

(3)

where V is the volume and N is the number of cationic sites of the unit cell, Xc is the optimal content of Bi3+ and Eu3+ ions. In the present work, V is 450.792 Å3, Xc is 0.28 and N is 8

[7]

. The Rc value calculated is 7.27 Å. Exchange interaction ET can be

excluded since it happens when the Rc is smaller than 5 Å. That is electric multipolar interaction plays a vital role in ET from Bi3+ to Eu3+ [1, 15]. Dexter’s theory was used to make clear the probable ET mechanism from Bi3+ to Eu3+. For multipolar-multipolar interaction, the next formula will be established [2],

S 0  C n /3 S

(4)

ηs0 and ηs represent the luminous quantum efficiencies of Bi3+ without and with Eu3+, respectively. C represents the sum of Bi3+ and Eu3+ content. The n value lies on interaction type. Particularly, n equal 6, 8 and 10 for dipole-dipole (d-d), dipole-quadrupole (d-q) and quadrupole-quadrupole (q-q) interactions, respectively. Because ηs0/ηs is not easy to acquire, it usually uses Is0/Is to estimate ηs0/ηs. Fig. 6 plots the relationship of Is0/Is versus Cn/3 based on above fact

[22]

. The best fitting could be

observed (99.915 %) only when n = 10. It indicates that q-q interaction dominants the ET from Bi3+ to Eu3+ in BaGd2O4 phosphors.

The energy level diagram of Bi3+ and Eu3+ in BaGd2O4 as well as electronic transitions with possible ET process are exhibited in Fig. 7. Excited by UV light (337 nm), electrons of Bi3+ are firstly pumped from ground 1S0 state to excited 3P1 state, and then relax nonradiatively to the lowest vibration mode of 3P1 state. At last Bi3+ at 3

P1 state radiatively relax down to ground 1S0 state, producing the typical blue band

emission centered at 432 nm. In BaGd2O4:0.08Bi3+,yEu3+ samples, because the excited 3P1 state of Bi3+ is close energetically to 5L6 (5D2) states of Eu3+, a resonance nonradiative ET process can be efficiently happened. An excited Bi3+ relaxes nonradiatively from excited level to ground level without emission of phonon and sends its excitation energy to a neighboring Eu3+, pumping it from ground 7F0 state to excited 5L6 (5D2) states. Such ET process will decline the emission intensity of Bi3+ (Fig. 4) and shorten the lifetime of Bi3+ emission (Fig. 5). Then the electrons of Eu3+ at 5L6 (5D2) levels will relax nonradiatively to the emitting 5D0 level. At last Eu3+ at 5D0 level relax radiatively to 7

FJ level, showing enhanced red emissions of Eu3+. Due to the efficient ET from Bi3+ to Eu3+, it is possible to achieve adjustable

emission from blue to red in BaGd2O4:0.08Bi3+,yEu3+ system by modulating Eu3+ content. Fig. 8 shows the CIE chromaticity diagram for BaGd2O4:0.08Bi3+,yEu3+ (λex = 337 nm) and BaGd2O4:0.2Eu3+ phosphors (λex = 394 nm). The CIE chromaticity coordinates move from blue, amaranth and orange red areas with the raising of Eu3+ content. And the CIE values for y = 0, 0.12 and 0.3 are (0.155, 0.079), (0.368, 0.204) and (0.548, 0.310). The CIE value for BaGd2O4:0.2Eu3+ sample is (0.564, 0.333). Notably, the dominating absorption of chlorophylls (380-480 nm, 600-700 nm) accords well with the blue emission of Bi3+ and red emission of Eu3+ (Fig. 4(a)), which means that BaGd2O4:0.08Bi3+,yEu3+ phosphors could convert UV light to blue and red light to boost the photosynthesis of plant [33, 44-46]. It should be mentioned that white light emission may be obtained by adding some green emission centers (such as Tb3+) in BaGd2O4:Bi3+,yEu3+ phosphors. Fig. 9(a) presents the temperature dependent emission spectra (λex = 337 nm) of BaGd2O4:0.08Bi3+,0.2Eu3+ phosphors from 303 to 528 K. It is obvious that the

emission intensities of Bi3+ and Eu3+ ions both decrease with raising temperature. Fig. 9(b) gives the integrated relative intensity dependence of Bi3+ and Eu3+ emissions on temperature. The intensities at 428 K for Bi3+ and Eu3+ remain only 51 and 37 % of the premier values at 303 K, respectively. Thermal quenching of emission intensity can be interpreted as follows. There is crosspoint between excited and ground states of active centers. The active centers from excited state can leap the crosspoint, and then relax nonradiatively to ground state, which results in lower emission intensity at higher temperature. The probability of such crosspoint nonradiative transition is forcefully rely on energy barrier (activation energy Ea). The following Arrhenius equation is used to estimate the activation energy [15, 22],  I  E ln  0   ln A  a kT  I (T ) 

(5)

where I0 and I(T) represent the integrated intensity at 303K and temperature T, respectively. k is Boltzmann constant (8.625×10−5 eV/K) and A is a constant. The plot of ln[(I0/I(T))−1)] versus 1/kT and the linear fit data are presented in Fig. 9(c). The fitted slope is -0.185, indicating Ea is about 0.185 eV. This relatively high Ea value manifests the good thermal characters of BaGd2O4:Bi3+,Eu3+ phosphors.

4. Conclusions In this work, a batch of Bi3+/Eu3+ single and co-doped BaGd2O4 phosphors were prepared by the conventional high temperature solid-state method. photoluminescent measurement

have

clarified

the

energy

transfer

from

Bi3+

to

Eu3+

in

BaGd2O4:Bi3+,Eu3+ phosphors is efficient, and quadrupole-quadrupole interaction may be the main energy transfer mechanism. Excited by 337 nm UV light, tunable emission including blue, amaranth and orange red emission can be obtained by tuning Eu3+ content in BaGd2O4:0.08Bi3+,yEu3+ phosphrs. The ability of converting UV light into blue and red light makes BaGd2O4:Bi3+,yEu3+ phosphors may be acted as promising candidate to boost the photosynthesis of plant.

Acknowledgments NSFC 11374269 and 11465010, the Major Discipline Academic and Technical Leaders Training Program of Jiangxi Province (20172BCB22023).

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Figure captions Fig. 1. XRD patterns (a) of BaGd2O4, BaGd2O4:0.08Bi3+, BaGd2O4:0.02Eu3+, BaGd2O4:0.08Bi3+,0.2Eu3+ and the standard data of BaGd2O4 (JCPDS NO. 82-2320) as a reference. SEM images of BaGd2O4:0.08Bi3+ (b). Fig. 2. (a) Excitation (λem = 432 nm) and (b) emission (λex = 337 nm) spectra of BaGd2O4:xBi3+ samples with different Bi3+ content. Fig. 3. (a) Spectra overlap between excitation (λem = 611 nm) of BaGd2O4:0.2Eu3+ and emission (λex = 337 nm) of BaGd2O4:0.08Bi3+, (b) excitation (λem = 611 nm) spectra of BaGd2O4:0.2Eu3+, BaGd2O4:0.08Bi3+, 0.2Eu3+ and excitation (λem = 432 nm) spectra of BaGd2O4:0.08Bi3+,0.2Eu3+. Fig. 4. (a) Emission spectra (λex = 337 nm) of BaGd2O4:0.08Bi3+,yEu3+, (b) emission intensities of Bi3+,Eu3+ and energy transfer efficiency as a function of Eu3+ concentration. Fig. 5. Decay curves of Bi3+ emission (λex = 337 nm, λem = 432 nm) in BaGd2O4:0.08Bi3+,yEu3+ phosphors. Fig. 6. Dependence of Is0/Is values on (a) C6/3, (b) C8/3 and (c) C10/3 in BaGd2O4:0.08Bi3+,yEu3+. Correlation efficiencies are 98.747, 99.775, 99.915 % for the fittings, respectively. Fig. 7. Energy-level diagram of Bi3+ and Eu3+ ions and energy transfer process. Fig. 8. CIE chromaticity coordinates of BaGd2O4:0.08Bi3+,yEu3+ (y = 0, 0.01, 0.02, 0.08, 0.12, 0.2, 0.3) (λex = 337 nm) and BaGd2O4:0.2Eu3+ phosphors, (λex = 394 nm). Fig. 9. (a) Temperature dependent emission spectra of BaGd2O4:0.08Bi3+,0.2Eu3+ (λex = 337 nm), (b) temperature dependent relative emission intensities of Bi3+ and Eu3+, (c) plot of ln(I0/IT −1) versus 1/kT and the linear fit of data through Eq. (5) for activation energy.

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