Site-Dependent Luminescence and Thermal Stability of Eu2+ Doped

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Site-dependent Luminescence and Thermal Stability of Eu2+ Doped Fluorophosphate towards White LEDs for Plant Growth Jiayu Chen, Niumiao Zhang, Chongfeng Guo, Fengjuan Pan, Xianju Zhou, Hao Suo, Xiaoqi Zhao, and Ewa M. Goldys ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b06102 • Publication Date (Web): 21 Jul 2016 Downloaded from http://pubs.acs.org on July 22, 2016

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ACS Applied Materials & Interfaces

Site-dependent Luminescence and Thermal Stability of Eu2+ Doped Fluorophosphate towards White LEDs for Plant Growth Jiayu Chena#, Niumiao Zhanga#, Chongfeng Guo*a,b, Fengjuan Panc, Xianju Zhoud, Hao Suoa, Xiaoqi Zhaoa, Ewa M. Goldysb a

National Key Laboratory of Photoelectric Technology and Functional Materials in Shaanxi Province, National

Photoelectric Technology and Functional Materials & Application of Science and Technology International Cooperation Base, Institute of Photonics & Photon-Technology and Department of Physics, Northwest University, Xi’an 710069, China; b

ARC Centre of Excellence for Nanoscale Biophotonics (CNBP), Macquarie University, North Ryde 2109,

Australia; c

MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, State Key Laboratory of Optoelectronic Materials

and Technologies, School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou 510275, China; d

School of Science, Chongqing University of Posts and Telecommunications, Chongqing, 400065, P. R. China.

* Author to whom correspondence should be addressed E-mail: [email protected] (Prof. Guo) Tel & Fax: ±86-29-88302661 #

These authors contributed equally to this work

Abstract: Eu2+ activated fluorophosphate Ba3GdNa(PO4)3F (BGNPF) with blue and red double-color emitting samples were prepared via a solid-state method in a reductive atmosphere. Their crystal structure and cationic sites were identified in light of X-ray diffraction pattern Rietveld refinement. Three different Ba2+ sites, coordinated by six O atoms referred to as Ba1, two F and five O atoms as Ba2 and two F and six O atoms as Ba3, were partially substituted by Eu2+. Photoluminescence emission (PL) and excitation (PLE) spectra of phosphor BGNPF: Eu2+ along well with the lifetimes were characterized at liquid helium temperature (LHT), which further confirm the existence of three Eu2+ emitting centers resulting in 436, 480, and 640 nm emission from 5d → 4f transitions of Eu2+ in three different Ba2+ crystallographic sites. These emissions overlap with the 1

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absorption spectra of carotenoids and chlorophylls from plants, which could directly promote the photosynthesis. Temperature-dependent PL spectra were used to investigate the thermal stability of phosphor, which indicates that the PL intensity of BGNPF: 0.9%Eu2+ with optimal composition at 150 ℃ still keep 60% its PL intensity at room temperature, in which blue emission has higher thermal-stability than the red emission. Furthermore, the approaching white LED devices have also been manufactured with a 365 nm n-UV LED chip and present phosphor, which make operators more comfortable than that of plant growth purple emitting LEDs system composed of blue and red light. Results indicate that this phosphor is an attractive dual-responsive candidate phosphor in the application n-UV light-excited white LEDs for plant growth.

Keywords: Phosphor; Site-dependent luminescence; LEDs; Plant growth; Thermal stability.

1. Introduction Light environment plays a key role as a major source of energy in plant cultivation and development. 1 However, not all of the sunlight could be absorbed by plants, and the basic need for light of plants growth distributes in blue (400 to 500 nm), red (600 to 690 nm) and far red (720 to 740

nm)

regions

are

accountable

to

phototropic

processes,

photosynthesis

and

photo-morphogenesis, respectively. 2-3 It is possible to enhance the output and traits of various crops by controlling the spectral composition to tune the plant growth process in phytotron chambers or greenhouses. In these conditions, the artificial lighting with the capability of spectral control and allows wavelengths to be matched with plant photoreceptors, makes it possible to influence plant morphology and composition.

4

Traditional gas discharge lamps (GDLs) are

usually used as light sources in green houses, but they are suboptimal for plant lighting because of the mismatch between their emission spectra and the absorption spectra of chlorophylls and carotenoids. 5 Light-emitting diodes (LEDs) present a versatile alternative for other horticultural light sources because they are engineered to elicit desirable plant responses through the control of the composition of phosphors. Moreover, they generally demonstrate high efficiency, long lifetime, environmental friendliness, small size and durability characteristics, which make the LEDs as the ideal illuminating system for plant cultivation. 6 The artificial lighting for plant growth has been fabricated by combining blue and red individual chips or using phosphor-converted light-emitting diodes (pc-LEDs).7 The former 2


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suffers from the spectrum mismatch with photosynthetic action spectrum and the high cost from the complicated circuit. The latter could be realized through exciting multi-phosphors using near ultraviolet (n-UV) or blue LED chips commonly available at low cost. However, the mixture with different ratios of several single-color emitting phosphors was usually used in pc-LEDs, which leads to low efficiency and shorter lifetime owing to the re-absorption of blue light by the red component and different aging rates for each phosphor. Multicolor-emitting phosphors are ideal choice to avoid such problems,8 including Eu2+ and Mn2+ co-doped A3MgSi2O8 (A = Ca, Sr, Ba), 3 Sr2Mg3P4O15

9

and SrMg2(PO4)2

10

phosphors emiting strong blue and red light, and Ce3+, Pr3+

co-doped Li2SrSiO4 phosphor emitting near infrared (NIR), red and blue light. 11 In general, the red and blue emissions of these phosphors depend on the energy migration process from the donor (Eu2+, Ce3+) to acceptor (Mn2+, Pr3+). The naturally weak Mn2+ d-d and Pr3+ f-f transitions usually cause additional excitation quenching routes to lower their thermal stability and quantum efficiency. 12 It is significant to develop a broad-band multicolor emitting phosphor doped with a single activator where the energy loss in the energy migration process is inhibited. This requires a host with several cationic sites available to be substituted by a dopant capable of eliciting variable colors in different crystal fields. Depending on the different strength of the crystal field of Eu2+occupied sites, the emission wavelength of Eu2+ ion arises from the 5d-4f transitions ranges from n-UV to near infrared region. Thus the Eu2+ ion is an ideal activator for exploring single-phased red and blue double color emitting phosphor for plant growth LEDs with a singly doped multicolor-emitting phosphor. Apatite type halo-phosphates with the general formula A10(PO4)6X2 are an important family to be serve as the phosphor hosts. Here A could be an alkaline metal, alkaline earth or a rare earth ion, and X generally stands for a halogen such as F, Cl and Br. 13 These compounds have been extensively investigated and found to have various composition, form diverse crystal field environments and excellent thermal stability. Importantly, they offer several different cationic sites for Eu2+.14-16 As a member of this family, Ba3GdNa(PO4)3F (BGNPF) is seldom used as the host of phosphor. Therefore, the structure, site-dependent spectra and thermal stability of Eu2+ doped Ba3GdNa(PO4)3F as well as the LED device were investigated in detail. Excitingly, BGNPF: Eu2+ phosphors show intense blue and red double color emission under the n-UV excitation, which can be absorbed by carotenoids and chlorophylls. Moreover, it exhibits nearly white emission under 3



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n-UV excitation. This makes the operating farmers more comfortable than the LEDs system generally used for plant growth composed of blue and red light. Our results presented in this paper indicate that as-prepared BGNPF doped with Eu2+ is an ideal candidate for white LEDs for the application in plant cultivation.

2. Experimental 2.1 Sample synthesis Eu2+ doped Ba3GdNa(PO4)3F sample was synthesized through a conventional solid reaction process and Eu2+ is supposed to enter the sites of Ba2+, thus the formula of sample are (Ba1-xEux)3GdNa(PO4)3F (BGNPF: xEu2+, x = 0.1-4%). Stoichiometric amounts of raw materials, including analytical grade BaCO3, BaF2 (excess 10 mol% to compensate the loss of fluorine), Na2CO3, NH4H2PO4, high purity (99.99%) rare earth oxides Gd2O3, Eu2O3 as well as a small quantity of flux NH4F, were weighed and mixed thoroughly. Subsequently, the acquired mixture were pre-sintered at 500 ℃ for 5 h and then calcined at 1080 ℃ for 3 h in CO reducing atmosphere. Finally, the naturally cooled sample were crushed to fine powder for further characterization. 2.2 Characterization of samples Powder X-ray diffraction (XRD) measurements were performed on a Rigaku MiniFlex600 diffractometer (Japan) with a 0.4 o/min scan speed ranging from 10o~110o using Cu Kα radiation with a wavelength of 1.5406 Å. The Rietveld refinement analysis and structure model were carried out using the Total Pattern Solution (TOPAS Academic) software. 17 The emission and excitation spectra at liquid helium temperature (LHT), temperature-dependent spectra and PL decay curves were obtained using an Edinburgh FLS920 spectrophotometer equipped with a 450 W Xe lamp as the excitation source for the steady state spectra and a microsecond flash-lamp (uF900H) as the pumping source for decay curves. Quantum efficiency was tested by the integrating sphere on the same Edinburgh FLS 920 fluorescence spectrometer combined with a 450 W Xe lamp as the excitation source and white BaSO4 is employed to be as a reference. An Oxford OptistatDN2 nitrogen cryogenics and CTI-Cryogenics temperature controlling system (with stable temperature over 0.5 h) were attached with FLS920 spectrometer to measure the temperature-dependent luminescent properties. 4


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3. Results and Discussion 3.1 Phase identification and structure analysis

Fig. 1 XRD patterns of the samples BGNPF: xEu2+ (x = 0, 0.1%, 0.7%, 0.9%, and 4%) and the standard profile of Ba3LaNa(PO4)3F (JCPDS 71-1317).

The phase purity and structure of the synthesized phosphors BGNPF: xEu2+ were identified using XRD. Figure 1 presents the XRD patterns of samples BGNPF: xEu2+ (x = 0, 0.1%, 0.7%, 0.9%, and 4%) as representatives because all samples show similar XRD patterns. It is apparently that all diffraction peaks of sample are consistent with those of the standard profile of Ba3LaNa(PO4)3F (BLNPE, JCPDS No. 71-1317) but slightly shift to large angle in comparison with those of Ba3LaNa(PO4)3F resulting from the smaller Gd3+ (r = 0.938 Å) ions substituting for the larger La3+ (r = 1.032 Å) ions, which implies that Ba3GdNa(PO4)3F: xEu2+ is isostructural with Ba3LaNa(PO4)3F and the doping of Eu2+ ions does not cause any significant impurities.

5



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Fig. 2 Rietveld refinement of XRD data of BGNPF: 0.9%Eu2+ by the TOPAS program. Calculated (red line) and experimental (crosses) XRD patterns of BGNPF: 0.9%Eu2+ phosphor. The green sticks stand for the positions of Bragg reflection and the blue line marks the difference between observed and calculated data. Table 1. Results of refinement and calculated crystallographic data for Ba2.973GdNa(PO4)3F: 0.9%Eu2+ sample. Ba2.973GdNa(PO4)3F: 0.9%Eu2+

Formula Space group

P -6 (No. 174), Hexagonal

a = b (Å)

9.7909

c (Å)

7.3185

α = β (deg.)

90

γ (deg.)

120

Z

2 3

V (Å )

607.58

Rwp (%)

6.815

Rp (%)

4.807

To further obtain more detailed information about the crystal structure of as-prepared sample and the site occupancy of the co-doping ions Eu2+ in the host, the Rietveld refinements of BGNPF and BGNPF: 0.9%Eu2+ were analyzed using the single crystal structure data of compound Ba3LaNa(PO4)3F (JCPDS No. 71-1317) as reference. Figure 2 shows the difference between the calculated (red) and experimental (crosses) XRD results as well as with their differences (blue) and Bragg positions for the refinement of BGNPF: 0.9%Eu2+ sample. The obtained residual 6


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factors (Rp = 4.807% and Rwp = 6.815%) of the refinement is low and the final refined crystallographic data are given in Table 1. The refinement results illuminate that BGNPF: 0.9%Eu2+ was a hexagonal lattice in the P-6 (No.174) space group, and the final lattice constants were calculated as a = b = 9.7909 Å, c = 7.3185 Å and V = 607.58 Å3. These values are smaller than those of the blank sample BGNPF: a = b = 9.8901 Å, c = 7.4684 Å and V = 617.43 Å3 from the Rietveld analysis (in Figure S1, Table S1 and S2) due to the substitution of Ba2+ with large radius by Eu2+ with small radius. These results again verify that the single-phased BGNPF is isotypic with BLNPE and Eu2+ ions have substituted the Ba2+ sites of the host without inducing a significant influence on the host structure.

Fig. 3 The crystal structure of Ba3GdNa(PO4)3F (a) and three kinds of Ba2+ ions with different coordination environment (b).

Figure 3 displays the spatial view of BGNPF crystal structure model together with the local coordination surroundings of Ba2+ ions. In the unit cell of Ba3GdNa(PO4)3F, it is highly disordered without symmetry center and the site number is similar with that of the compound Ba3LaNa(PO4)F. Three types of Ba2+ crystallographic sites (Ba1, Ba2, and Ba3), two Gd sites (Gd1 and Gd2), two P sites (P1 and P2), and two F sites (F1 and F2) were observed in the host lattice. 18-21 The coordination polyhedron of Ba1 is coordinated by six oxygen atoms, the coordination number (CN) is 6; five O and two F atoms for Ba2 (CN = 7) and six O and two F atoms for Ba3 (CN = 8), as 7



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shown in Figure 3b. These cation polyhedrons are disordered in orientation, connecting to each other by the tetrahedral (P2O4)3- groups; the F1 and F2 sites are commutatively parallel to c axis. Taking into consideration refinement results of the lattice constants, identical valency and the closer ionic radii between Ba2+ (1.35, 1.38, and 1.42 Å for CN = 6, 7, and 8, respectively) and Eu2+ (1.31, 1.34, and 1.39 Å for CN = 6, 7, and 8, respectively) ions, 22 it is reasonable to deduce that Eu2+ ions distribute in the Ba1, Ba2 and Ba3 sites and result in three different Eu2+- emitting centers in BGNPF: Eu2+ phosphors. 3.2 Photoluminescence of BGNPF: Eu2+ at room temperature

Fig. 4 PL and PLE spectra of the phosphor BGNPF: 0.9%Eu2+ along with the absorption spectra of chlorophyll a, b and carotenoids (a), blue and red emission intensity as function of Eu2+ concentration (b) and linear fit of In(x) versus In(I/x) for the blue and red emitting band centered at 472 and 608 nm in BGNPF: xEu2+ phosphors (c).

Figure 4a exhibits the PLE and PL spectra of the phosphor BGNPF：0.9% Eu2+. The excitation spectra monitored at 472 and 608 nm both consist of broad absorption bands ranging from 250-450 nm centered at 342 nm, attributed to the 4f7 → 4f65d1 transitions of Eu2+. The PL spectrum clearly includes two intense broad emitting bands in blue region peaked at 472 nm and red region centered at 608 nm with 342 nm UV-light excitation. These emissions originate from the allowed electronic transitions of 4f65d1 → 4f7 of Eu2+ in different crystallographic sites of BGNPF.

23

Obviously, this phosphor could be efficiently excited by an n-UV LED chip and

provide red and blue components to LEDs. Comparing with the absorption spectra of chlorophyll a, chlorophyll b and carotenoid responsible for photosynthesis of plants (in Figure 4a), a 8


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significant overlap with the PL spectrum of BGNPF: Eu2+ was found, implying that BGNPF: Eu2+ could be used in plant illumination LEDs to accelerate the plant growth and development. A series of samples BGNPF: xEu2+ with different activator concentration have been prepared to determine the optimal composition. Figure 4b shows the variable PL intensities of blue and red emission as a function of Eu2+ contents, in which their PL intensities gradually increase before reaching the maximum at x = 0.7% for blue emission and x = 0.9% for red emission, respectively. With the contents of Eu2+ beyond the critical concentration, their PL intensities begin to fall because of the concentration quenching effect resulting from the non-radiative energy migration among the identical activator Eu2+ ions. Generally, non-radiative energy migration was attributed to radiation re-absorption, exchange interaction or electric multipolar interaction. It appears that the energy transfer among Eu2+ in the BGNPF phosphors should not be controlled by the exchange interaction which commonly occurs in a forbidden transition; the radiation re-absorption mechanism seems also impossible since no obvious overlap was found between their PLE and PL spectrum. Thus, the electric multi-polar interaction will responsible for the non-radiative concentration quenching between two nearest Eu2+ centers. Based on the Dexter’s model, the mechanism of interaction is determined by the Eq. (1): 24, 25 θ   I = K 1 + β ( x ) 3  x  

−1

(1)

where I and x represent the PL emission intensity and the concentration of activator ion bigger than the critical concentration, respectively; ß and K are specific constants for a given host crystal and excitation condition. θ = 3, 6, 8 or 10 denotes the non-radiative energy transfer mechanism of exchange coupling, dipole-dipole, dipole-quadrupole and quadrupole-quadrupole interactions, respectively. The fitting lines of In(I/x) vs. In(x) for two emission bands centered at 472 and 608 nm (in in Figure 4c), demonstrates that the slopes of two fitting lines are -1.76 and -1.83, respectively. Therefore, the values of θ are 5.28 and 5.49, close to 6, implying that the main concentration quenching mechanism of Eu2+ in the 472 and 608 nm emission centers in the BGNPF host are both the electric dipole-dipole interactions. The quantum efficiency of Ba3GdNa(PO4)3F: 0.9%Eu2+ was about 62.4% with 365 nm excitation, which is superior to those of Eu2+-Mn2+codoped samples.

26, 27

The above results prove that Ba3GdNa(PO4)3F:Eu2+ phosphor

is an outstanding candidate for white light-emitting diodes. 9



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3.3 Site-dependent photoluminescence of BGNPF: Eu2+ at low temperature

Fig. 5 (a) PL (λex = 342 nm) spectra and Gaussian fitting of three different Eu2+ ions center, (b) PLE (λem = 436, 480 and 640 nm) spectra together with (c) decay curves of Eu2+ at different sites in BGNPF: 0.9%Eu2+ at LHT.

On the basis of the refinement result, there are three Ba2+ sites with differently coordinated surroundings, it is reasonable for Eu2+ to occupy any Ba2+ sites in BGNPF, generating three emission bands because of the crystal field environment dependent Eu2+ ions emission. However, only two emission bands were found in BGNPF: Eu2+ at room temperature, which is distinctly different from our deduction. To further confirm the occupancy of Eu2+ in BGNPF: 0.9%Eu2+, the emission and excitation spectra of which were measured at liquid helium temperature (LHT) and displayed in Figure 5. Under 342 nm excitation, the emission spectrum could be decomposed into three Gaussian peaks A, B and C centered at 436, 480 and 640 nm, respectively. For the PLE spectra monitored at 436, 480 and 640 nm, they are distinctively different, as shown in Figure 5b, due to the emission bands coming from different Eu2+ emitting centers. The lifetimes of Eu2+ at different sites are different, thus it is also an efficient method to identify the sites of Eu2+ by measuring their decay curves. Figure 5c exhibits the decay curves of Eu2+ at LHT in compound BGNPF upon 342 nm excitation and monitored at 436, 480 and 640 nm at LHT, respectively. The single exponential was used to fit the decay curves, and the effective lifetimes can be obtained by the following Eq. (2):28, 29

10


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∞

∫ I (t)tdt

τ = 0∞

(2)

∫ I (t)dt 0

where I(t) is the emission intensity as the time is t. The effective lifetime for the emission of 436, 480 and 640 nm were calculated to be about 0.44, 0.54 and 1.88 µs, respectively, which implies that three emission bands are attributed to three different Eu2+ ions emitting centers in BGNPF: Eu2+ phosphor. The above results confirm that Eu2+ ions enter three different Ba2+ sites resulting in three different broad band emissions centered at 436, 480 and 640 nm in BGNPF: Eu2+ phosphor. It is also important to identify the coordination surroundings of Eu2+ emitting at different wavelength. Based on the crystal field theory, the type of polyhedron and its ligand are main factors to determine the crystal field strength of the three Ba2+ sites occupied by Eu2+. The stronger degree of crystal field splitting of Eu2+ leads to lowering of the 5d band of Eu2+ and of the emission energy. The crystal field around Eu2+ satisfies Eq. (3): 30

Dq =

ze 2 r 4 6 R5

(3)

where Dq is measurement of the crystal field strength, z is the charge or valence of the anion, R is the distance between the central ion and its ligands, r is the radius of the d wave-function and e is the charge of an electron. According to the refinement results, the Eu2+ ions occupy the three Ba2+sites with different coordination numbers and bond distances in the crystal lattice of BGNPF: 0.9%Eu2+. The bond distances of Ba/Eu-O/F in BGNPF: 0.9%Eu2+ sample are listed in Table S3, and the average bond distances of Ba1/Eu1-O, Ba2/Eu2-O, Ba2/Eu2-F, Ba3/Eu3-O and Ba3/Eu3-F are 2.4678, 2.4637, 2.4287, 2.8368 and 2.6902 Å, respectively. Based on the Eq. (3), the emission band will shift towards long wavelength resulting from doping Eu2+ into Ba2+ sites with stronger crystal field strength (shorter Ba−O/F bond distance). In addition, the emission position of Eu2+ can be simply estimated in accordance with Van Uitert Eq. (4), which creates the relationship between the coordinate environment and emission peaks and successfully explain many structure-property relationships in Eu2+ and Ce3+ doped systems: 31, 32

11



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1  nEar  V − V   E = Q 1 −   ×10 80   4   

Page 12 of 22

(4)

in which E is the energy location of the lower d-band edge for Eu2+ (cm-1); Q is the energy location for the lower d-band edge for the free ion (Q = 34000 cm-1 for Eu2+), V is the valence of the active cation (V = 2 for Eu2+), n is the coordination number, r is the radius of the host cation replaced by the activator Eu2+ ion (Å). The value of Ea is difficult to obtain due to the complexity of the host BGNPF, but it is constant in the same host. E is proportional to the product of n and r, and could be qualitatively estimated based on above analysis. In the present case, Eu2+ ions occupy three Ba sites with CN = 6 by six O (Ba1, r = 1.35 Å), 7 by five O and two F (Ba2, r = 1.38 Å) and 8 by six O and two F (Ba3, r = 1.42 Å), 22 respectively. The emission wavelength of Eu2+ shifts to the red region with the decrease of CN value of the substituted Ba2+. Therefore, the emission band A peaked at 436 nm, band B peaked at 480 nm and band C peaked at 640 nm are ascribed to the 5d → 4f transitions of Eu2+ in the Ba3 site with eight-coordinated, Ba2 site with seven-coordinated and the Ba1 site with six-coordinated, respectively, which is in agreement with estimation based on Eq. (3). However, an extra weak shoulder band emission at 436 nm from Eu2+ occuping the sites of Ba3 disappeared at the room temperature, only two stronger emission centered at 480 and 640 nm from the sites Ba2 and Ba1 reemerged; which implies that the emission of Eu2+ from the sites of Ba3 is quenched at or above room temperature. 3.4 Thermal stability of sample BGNPF: Eu2+ For the performance of phosphors applied in LEDs, especially for LEDs with high power, the thermal-stability of phosphor is a key parameter because its working temperature could arrive at 150 ℃, which seriously affect the performance of LEDs, such as the light output, lifetime, chromaticity and color rendering index. To evaluate the thermal quenching behavior of BGNPF: 0.9%Eu2+, the PL spectra of BGNPF: 0.9%Eu2+ at different temperatures heating from 298 to 498 K and cooling from 498 to 298 K with the excitation of 342 nm light were shown in Figure 6a and b, respectively. For the heating process, the PL intensity of BGNPF: 0.9% gradually decreases with increasing temperature, whereas for the cooling process, its PL intensity grows with the decrease of temperature. The normalized integrated PL intensity was displayed in Figure 6 as function of temperature, in which the emission intensity of sample at 423 K (150 ℃) keeps 12


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approximately 60% of that at 298 K. Meanwhile, thermal quenching temperature T0.5, at which the emission intensity decreases to 50% of its original 33, is 442 K for BGNPF: 0.9%Eu2+ phosphor. Comparing the heating and cooling process, no obvious changes were found for their emission spectra and the temperature-dependent intensity variable curves could repeat exactly, indicating that the present sample offers the reversible thermal behavior and excellent thermal stability. However, the decreasing rate of PL intensity in red region is faster than that of PL intensity in blue region, which further demonstrates that red and blue emission comes from different emitting centers in Eu2+ doped BGNPF systems.

Fig. 6 Normalized PL intensity of BGNPF: 0.9%Eu2+. Insets: temperature-dependence of PL spectra of BGNPF: 0.9%Eu2+ (a) from 298 to 498 K, (b) then cooled from 498 to 298 K.

To better comprehend the thermal quenching behavior and estimate the value of its activation energy, the Arrhenius equation was employed to analyze the thermal quenching data of BGNPF: 0.9%Eu2+ in blue and red region as shown in Figure 7, respectively. The temperature quenching mechanism is also illuminated by a simple schematic configuration coordinate diagram in Figure 7, the ground (4f76s25d0) and two excited states of Eu2+ were described with parabola and two overhead parabolas, in which the excited states (4f66s05d1) located at different energy levels due to their different coordination surroundings and crystal field strengths. The electron at excited states could be excited thermally with energy (Ea) to reach the intersection between the excited state and the ground state in the configurational coordinate diagram. Then these electrons back to ground 13



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states with heat dissipation instead of radiation emission, resulting in thermal quenching of luminescence and phonon emission. Here, the value of the activation energy is a crucial parameter to thermal quenching effect, which is described by Eq. (5): 34-36

IT =

I0 Ea 1 + c exp(− ) kT

(5)

Where I0 and IT are the intensities of the initial and different temperatures, respectively, c is a rate constant, Ea is the activation energy, and k is the Boltzmann constant (8.629 x 10-5 eV K-1). Figure 7 shows the In(I0/IT-1) vs. 10000/T lines for two emission bands centered at 472 and 608 nm in BGNPF: 0.9Eu2+ phosphor. According to the Eq. (5), the slopes of two fitting line are equal to -0.41738 and -0.26563, respectively. The activation energy was calculated to be Ea1= 0.36 for peak at 472 nm and Ea2 = 0.23 eV for peak at 608 nm.

Fig. 7 Activation energy of thermal quenching for emission band 472 and 608 nm in BGNPF: 0.9%Eu2+ phosphors. The inset shows the schematic configuration coordinate diagram of the Eu2+ ion.

The probability that a non-radiative transition occurs during per unit time (α) can be defined according to the Eq. (6): 37

α = s exp(−

Ea ) kT

(6)

where s is the frequency factor (s-1), k is the Boltzmann constant, and T is the temperature. It is 14


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clear that lower activation energy leads to a greater probability of nonradiative transition. According to above results, Ea1 > Ea2, α1 (process ①) < α2 (process ②), as shown in inset of Figure 7, the barrier Ea2 can be easily overcome to produce nonradiative transition. Therefore, high-energy emission (472 nm) dominates the PL spectra as the increase of temperature, which implies that emission of Eu2+ at Ba2 shows better thermal stability than that of Eu2+ at Ba1 sites. This result agrees well with previous conclusion: Activation energy increases with the decrease of Stokes shift and the smaller Stokes shift leads to lower thermal luminescence quenching. 38, 39 In addition, the blue and red peak positions of emission spectra of sample BGNPF: 0.9Eu2+are found shift from 472, 608 nm at room temperature to 480, 640 nm at LHT. The reason for this is that the electrons at the excited state with low-energy or long-wavelength emission are thermally excited with the assistance of phonons to reach the excited state with high-energy or short-wavelength emission via the intersection between excited states in the configuration coordinate diagram.

40

When the temperature is lower, the electrons at the excited state1 can overcome the energy barrier ∆E1 (process 2) to reach the excited state2, thus the red emission increases. When the temperature is higher, the back-transfer overcome the energy barrier ∆E2 is feasible (process 1), which will induce blue emission increases. Hence, the blue-shift appears at high temperature in comparison with that of at LHT. 3.5 Fabrication of White LEDs The emission color of the present phosphors BGNPF: Eu2+ changed with various Eu2+ contents because its emission intensity in blue and red strongly depend on the Eu2+ concentration. Thus the corresponding CIE chromaticity coordinates of samples varied from blue (0.190, 0.215) for x = 0.1%, through cyan (0.229, 0.241) for x = 0.7% to white (0.273, 0.275) for x = 0.9%, which indicates that the emission color of BGNPF: Eu2+ could be randomly controlled by adjusting Eu2+ contents. With the purpose of investigating the application of BGNPF: Eu2+ in white LEDs, a phosphor-converted LED device was made using BGNPF: 0.9%Eu2+ with a near ultraviolet InGaN chip (365 nm). Figure 8 exhibited the electroluminescence (EL) spectrum of the LED device with 25 mA current, in which only one blue band peaked 472 nm at and a red band peaked at 608 nm from Eu2+ emission in BGNPF: 0.9%Eu2+ were found. The prototype of LED and the CIE of samples were shown in the inset of Figure 8, point 1-3 stand for that of sample BGNPF: xEu2+

(x

=

0.1%,

0.7%,

0.9%),

respectively.

The

fabricated

15


BGNPF:

0.9%Eu2+


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phosphor-converted LED emits white light with the color coordinate of (0.273, 0.275), color rendering index (Ra) of 81 and correlated color temperature (CCT) of 5402 K. All above results further prove that the BGNPF: Eu2+ phosphor can be excited by n-UV LED chips to emit an approaching white light.

Fig. 8 Electroluminescence spectrum of LED device combining an n-UV InGaN LED chip (365 nm) with BGNPF: 0.9%Eu2+ phosphor. Inset shows CIE chromaticity coordinates for BGNPF: xEu2+ and the photograph of the fabricated w-LED package.

4. Conclusions Eu2+ doped BGNPF single-component with double-color emission were successfully prepared by solid-state method in CO reduction atmosphere, exhibiting a broad blue emission centered at 472 nm and a broad red emission centered at 608 nm with the excitation of n-UV light. The detailed crystal structure of Eu2+ doped BGNPF was investigated via the XRD Rietveld refinements, indicating that three Ba sites are coordinated with six O atoms (Ba1), two F and five O atoms (Ba2) and two F and six O atoms (Ba3), respectively. Emission spectra and lifetimes at LHT have been used to identify that Eu2+ enter three Ba2+ sites, resulting three band emission centered at 436, 480, and 640 nm from the 4f65d1 → 4f7 transitions of Eu2+ in crystallographic sites Ba3 (CN = 8), Ba2 (CN = 7) and Ba1 (CN = 6), respectively. However, the emission centered at 436 nm is quenched at room temperature, only blue and red emissions centered at 472 and 608 nm appear, respectively. 16


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The site-dependent thermal stabilities of 472 and 608 nm were studied based on the heating and cooling temperature-dependent PL spectra with 342 nm excitation. Results indicate that the blue emission at 472 nm is more stable than that of red one owing to high thermal activation energy. The approaching white LEDs were fabricated by using 365 nm n-UV chips to excite BGNPF: 0.9%Eu2+ phosphor and EL spectrum includes a blue and a red two broad band emissions peaked at 472 and 608 nm, respectively, which overlap with the absorption spectra of chlorophylls and carotenoids of plants. In addition, the values of color coordinates, CCT and CRI for the fabricated LEDs are (0.273, 0.275), 5402 K and 81, respectively, which is better to operator working in greenhouse than that of purple LED. Results indicate that the single-component phosphor BGNPF: Eu2+ with blue and red double-color emission possesses an outstanding potential in the application of white LEDs for plant growth.

Acknowledgments: This work was supported by National Natural Science Foundation of China (No. 11274251), Research Fund for the Doctoral Program of Higher Education of China (RFDP) (No.20136101110017), Natural Science Foundation of Shaanxi Province (No.2014JM1004), Foundation of Shaanxi Province Educational Department (15JS101, 15JK1712) and Foundation of Key Laboratory of Photoelectric Technology in Shaanxi Province (12JS094).

Supporting Information Rietveld refined XRD patterns; crystallographic data; lattice parameters and atom positions data; Bond distances data.

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References [1] Agarwal, A.; Gupta, S. D. Impact of Light-Emitting Diodes (LEDs) and Its Potential on Plant Growth and Development in Controlled-Environment Plant Production System. Curr. Biotechnol. 2016, 5, 28-43. [2] Tamulaitis, G.; Duchovskis, P.; Bliznikas, Z.; Breivė, K., Ulinskaitė, R.; Brazaitytė, A.; Novičkovas, A.; Žukauskas, A. High-Power Light-Emitting Diode Based Facility for Plant Cultivation. J. Phys. D: Appl. Phys. 2005, 38, 3182-3187. [3] Ma, L.; Wang, D. J.; Mao, Z. Y., Lu, Q. F.; Yuan, Z. H. Investigation of Eu–Mn Energy Transfer in A3MgSi2O8: Eu2+, Mn2+ (A = Ca, Sr, Ba) for Light-Emitting Diodes for Plant Cultivation. Appl. Phys. Lett. 2008, 93, 144101. [4] Olle, M.; Viršilė, A. The Effects of Light-Emitting Diode Lighting on Greenhouse Plant Growth and Quality. Agric. Food Sci. 2013, 22, 223-234. [5] Mao, Z. Y.; Chen, J. J.; Li, J.; Wang, D. J. Dual-Responsive Sr2SiO4: Eu2+-Ba3MgSi2O8: Eu2+, Mn2+ Composite Phosphor to Human Eyes and Plant Chlorophylls Applications for General Lighting and Plant Lighting. Chem. Eng. J. 2016, 284, 1003-1007. [6] Massa, G. D., Kim, H.; Wheeler, R. M.; Mitchell, C. A. Plant Productivity in Response to LED Lighting. HortScience 2008, 43, 1951-1956. [7] Anželika, K.; Renata, M.; Stasė, D.; Silva, Ž.; Genakij, K.; Gintautas, T.; Pavelas, K.; Artūras, Ž. In Vitro Culture of Chrysanthemum Plantlets Using Light-Emitting Diodes. Cent. Eur. J. Biol. 2008, 3, 161-167. [8] Guo, C. F.; Xu, Y.; Ren, Z. Y.; Bai, J. T. Blue-White-Yellow Tunable Emission from Ce3+ and Eu2+ Co-Doped BaSiO3 Phosphors. J. Electrochem. Soc. 2011, 158, J373-J376. [9] Guo, C. F.; Ding, X.; Luan, L.; Xu, Y. Two-Color Emitting of Eu2+ and Mn2+ Co-Doped Sr2Mg3P4O15 for UV LEDs. Sens. Actuators B 2010, 143, 712-715. [10] Guo, C. F.; Luan, L.; Ding, X.; Huang, D. X. Luminescent Properties of SrMg2(PO4)2: Eu2+, Mn2+ as a Potential Phosphor for Ultraviolet Light-Emitting Diodes. Appl. Phys. A: Mater. Sci. Process. 2008, 91, 327-331. [11] Chen, J. Y.; Guo, C. F.; Yang, Z.; Li, T.; Zhao, J. Li2SrSiO4: Ce3+, Pr3+ Phosphor with Blue, Red, and Near-Infrared Emissions Used for Plant Growth LED. J. Am. Ceram. Soc. 2016, 99, 218-225. [12] Dai, P. P.; Li, C.; Zhang, X. T.; Xu, J.; Chen, X.; Wang, X. L.; Jia, Y.; Wang, X. J.; Liu, Y. C. A Single Eu2+-Activated High-Color-Rendering Oxychloride White-Light Phosphor for White-Light-Emitting Diodes. Light: Sci. Appl. 2016, 5, 16024. [13] Li, K.; Geng, D. L.; Shang, M. M.; Zhang, Y.; Lian, H. Z.; Lin, J. Color-Tunable Luminescence and Energy Transfer Properties of Ca9Mg (PO4)6F2: Eu2+, Mn2+ Phosphors for UV-LEDs. J. Phys. Chem. C 2014, 118, 18


Page 18 of 22

Page 19 of 22

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


11026-11034. [14] Jiao, M. M.; Jia, Y. C.; Lu, W.; Lv, W. Z.; Zhao, Q.; Shao, B. Q.; You, H. P. Sr3GdNa(PO4)3F: Eu2+, Mn2+: a Potential Color Tunable Phosphor for White LEDs. J. Mater. Chem. C 2014, 2, 90-97. [15] Guo, C. F.; Luan, L.; Ding, X.; Zhang, F. J.; Shi, F. G.; Gao, F.; Liang, L. F. Luminescent Properties of Sr5(PO4)3Cl: Eu2+, Mn2+ as a Potential Phosphor for UV-LED Based White LEDs. Appl. Phys. B: Lasers Opt. 2009, 95, 779-785. [16] Yu, J.; Guo, C. F.; Ren, Z. Y.; Bai, J. T. Photoluminescence of Double-Color-Emitting Phosphor Ca5(PO4)3Cl: Eu2+, Mn2+ for Near-UV LED. Opt. Laser Technol. 2011, 43, 762-766. [17] Coelho, A. A. TOPAS Academic V5, Coelho Software: Brisbane, Australia, 2005. [18] Jiao, M. M.; Guo, N.; Lu, W.; Jia, Y. C.; Lv, W. Z.; Zhao, Q.; Shao, B. Q.; You, H. P. Tunable Blue-Green-Emitting Ba3LaNa(PO4)3F: Eu2+,Tb3+ Phosphor. Inorg. Chem. 2013, 52, 10340-10346. [19] Mi, R. Y.; Zhao, C. L.; Xia, Z. G. Synthesis, Structure, and Tunable Luminescence Properties of Novel Ba3NaLa(PO4)3F: Eu2+, Mn2+ Phosphors. J. Am. Ceram. Soc. 2014, 97, 1802-1808. [20] Zeng, C. Hu, Y.; Xia, Z. G.; Huang, H. W. A Novel Apatite-Based Warm White Emitting Phosphor Ba3GdK(PO4)3F: Tb3+, Eu3+ with Efficient Energy Transfer for W-LEDs. RSC Adv. 2015, 5, 68099-68108. [21] Zeng, C., Liu, H. K.; Hu, Y.; Liao, L. B.; Mei, L. F. Color-Tunable Properties and Energy Transfer in Ba3GdNa(PO4)3F: Eu2+, Tb3+ Phosphor Pumped for N-UV W-LEDs. Opt. Laser Technol. 2015, 74, 6-10. [22] Shannon, R. D. Revised Effective Ionic Radii and Systematic Studies of Interatomic Distances in Halides and Chalcogenides. Acta Crystallogr. 1976, 32, 751-767. [23] Guo, C. F.; Xu, Y.; Ding, X.; Li, M.; Yu, J.; Ren, Z. Y.; Bai, J. T. Blue-Emitting Phosphor M2B5O9Cl: Eu2+ (M = Sr, Ca) for White LEDs. J. Alloys Compd. 2011, 509, L38-L41. [24] Huang, C.; Chiu, Y.; Yeh, Y.; Chan, T.; Chen, T. Eu2+-Activated Sr8ZnSc(PO4)7: a Novel Near-Ultraviolet Converting Yellow-Emitting Phosphor for White Light-Emitting Diodes. ACS Appl. Mater. Interfaces 2012, 4, 6661-6668. [25] Wang, B.; Lin, H.; Xu, J.; Chen, H.; Wang, Y. CaMg2Al16O27: Mn4+-Based Red Phosphor: a Potential Color Converter for High-Powered Warm W-LED. ACS Appl. Mater. Interfaces 2014, 6, 22905-22913. [26] Miao, S. H.; Xia, Z. G.; Zhang, J.; Liu, Q. L. Increased Eu2+ Content and Codoping Mn2+ Induced Tunable Full Color Emitting Phosphor Ba1.55Ca0.45SiO4:Eu2+, Mn2+. Inorg. Chem. 2014, 53, 10386-10393. [27] Lee, K. H.; Choi, S.; Jung, H. K.; Im, W. B. Bredigite-Structure Ca14Mg2(SiO4)8: Eu2+, Mn2+: A Tunable Green-Red-Emitting Phosphor with Efficient Energy Transfer for Solid-State Lighting. Acta Mater. 2012, 60, 19



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

5783-5790. [28] Suo, H.; Guo, C. F.; Li, T. Broad-Scope Thermometry Based on Dual-Color Modulation Up-Conversion Phosphor Ba5Gd8Zn4O21: Er3+/Yb3+. J. Phys. Chem. C 2016, 120, 2914-2924. [29] Zhou, X. J.; Zhao, X. Q.; Wang, Y. J.; Wu, B.; Shen, J.; Li, L.; Li, Q. X. Eu (III) and Tb (III) Complexes with the Nonsteroidal Anti-Inflammatory Drug Carprofen: Synthesis, Crystal Structure, and Photophysical Properties. Inorg. Chem. 2014, 53, 12275-12282. [30] Huang, C. H.; Chen, T. M. Ca9La(PO4)7: Eu2+, Mn2+: an Emission-Tunable Phosphor Through Efficient Energy Transfer for White light-Emitting Diodes, Opt. Express 2010, 18, 5089-5099. [31] Wang, X. J.; Wang, L.; Takeda, T.; Funahashi, S.; Suehiro, T.; Hirosaki, N.; Xie, R. Blue-Emitting Sr3Si8–xAlxO7+ xN8–x: Eu2+ Discovered by a Single-Particle-Diagnosis Approach: Crystal Structure, Luminescence, Scale-Up Synthesis, and Its Abnormal Thermal Quenching Behavior. Chem. Mater. 2015, 27, 7689-7697. [32] Van Uitert L. G. An Empirical Relation Fitting the Position in Energy of the Lower d-band Edge for Eu2+ or Ce3+ in Various Compounds. J. Lumin. 1984, 29, 1-9. [33] Parmentier, A. B.; Smet, P. F.; Bertram, F.; Christen J.; Poelman, D. Structure and Luminescence of (Ca, Sr)2SiS4: Eu2+ Phosphors. J. Phys. D: Appl. Phys. 2010, 43, 085401. [34] Zhang, N. M.; Guo, C. F.; Zheng, J. M.; Su, X. Y.; Zhao, J. Synthesis, Electronic Structures and Luminescent Properties of Eu3+ Doped KGdTiO4. J. Mater. Chem. C 2014, 2, 3988-3994. [35] Lee, S.; Chan, T.; Chen, T. Novel Reddish-Orange-Emitting BaLa2Si2S8: Eu2+ Thiosilicate Phosphor for LED Lighting. ACS Appl. Mater. Interfaces 2014, 7, 40-44. [36] Xia, Z. G.; Zhou, J.; Mao, Z. Y. Near UV-Pumped Green-Emitting Na3(Y, Sc)Si3O9: Eu2+ Phosphor for White-Emitting Diodes. J. Mater. Chem. C 2013, 1, 5917-5924. [37] Lv, W. Z.; Jia , Y. C.; Zhao, Q.; Jiao, M. M.; Shao, B. Q.; Lü, W.; You, H. P. Crystal Structure and Luminescence Properties of Ca8Mg3Al2Si7O28: Eu2+ for WLEDs. Adv. Opt. Mater. 2014, 2, 183-188. [38]

Li, G. G.; Lin, C. C.; Chen, W.; Molokeev, M. S.; Atuchin, V. V.; Chiang, C.; Zhou, W. Z.; Wang, C.; Li,

W.; Sheu, H.; Chan, T.; Ma, C. G.; Liu, R. Photoluminescence Tuning via Cation Substitution in Oxonitridosilicate Phosphors: DFT Calculations, Different Dite Occupations, and Luminescence Mechanisms. Chem. Mater. 2014, 26, 2991-3001. [39] Liu, L. H.; Xie, R.; Hirosaki, N.; Takeda, T.; Li, J. G.; Sun, X. D. Temperature Dependent Luminescence of Yellow-Emitting a-Sialon: Eu2+ Oxynitride Phosphors for White Light-Emitting Diodes. J. Am. Ceram. Soc. 2009, 92, 2668-2673. 20


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[40] Kim, J. S.; Park, Y. H.; Kim, S. M.; Choi, J. C.; Park, H. L. Temperature-Dependent Emission Spectra of M2SiO4: Eu2+ (M = Ca, Sr, Ba) Phosphors for Green and Greenish White LEDs. Solid State Commun. 2005, 133, 445-448.

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Graphical Abstract

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Site-Dependent Luminescence and Thermal Stability of Eu2+ Doped

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