High-efficient and thermal-stable Ca19Zn2(PO4)14: Eu2+, Mn2+ blue-red dual-emitting phosphor for plant cultivation LEDs

High-efficient and thermal-stable Ca19Zn2(PO4)14: Eu2+, Mn2+ blue-red dual-emitting phosphor for plant cultivation LEDs

Journal of Alloys and Compounds 811 (2019) 151956 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: http:/...

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Journal of Alloys and Compounds 811 (2019) 151956

Contents lists available at ScienceDirect

Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom

High-efficient and thermal-stable Ca19Zn2(PO4)14: Eu2þ, Mn2þ blue-red dual-emitting phosphor for plant cultivation LEDs Zishan Sun a, 1, Zhenpeng Zhu a, 1, Jiabao Luo a, Ziying Guo a, Xinguo Zhang a, *, Zhan-chao Wu b a

Guangdong Provincial Key Laboratory of New Drug Screening, School of Pharmaceutical Sciences, Southern Medical University, Guangzhou, 510515, China State Key Laboratory Base of Eco-chemical Engineering, Laboratory of Inorganic Synthesis and Applied Chemistry, College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology, Qingdao, 266042, China b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 3 April 2019 Received in revised form 2 August 2019 Accepted 21 August 2019 Available online 22 August 2019

A series of dual-emitting Eu2þ, Mn2þ co-doped Ca19Zn2(PO4)14 (CZP) phosphors capable for plant cultivation LEDs are synthesized. As a sensitzer, Ca19Zn2(PO4)14: Eu2þ emits a dominant blue light with green shoulder by Eu2þ occupying CaO8 and CaO6 sites, respectively. Mn2þ was introduced into CZP: Eu2þ to obtain the red emission component. The as-synthesized CZP: Eu2þ, Mn2þ phosphor shows a blue emission peaking at 415 nm and significantly enhanced deep-red emission peaking at 650 nm under single UV excitation, which should be attributed to the existence and energy transfer of Eu2þ and Mn2þ, respectively. The emission spectra match well with the plants’ photosynthetic action spectra (PAS) for both red and blue lights simultaneously. Energy transfer from Eu2þ to Mn2þ enables a facile tune of emitting color from blue to red, and the corresponding mechanism is revealed to be the electric dipoledipole interaction by analyzing Eu2þ/Mn2þ decay lifetime variations. High quantum efficiency (93.3%) and favorable thermal stability (68% of RT intensity at 473 K) clearly verify the good properties of CZP: Eu2þ, Mn2þ phosphor. Results confirmed that CZP: Eu2þ, Mn2þ phosphor might have great applicational potential in plant growth LED lighting. © 2019 Elsevier B.V. All rights reserved.

Keywords: Phosphor Eu2þ-Mn2þ Plant cultivation LEDs Energy transfer

1. Introduction Light provides numerous energy source for plants growth, as well as controls the growing rhythms of many procedures, such as seed germination, seedling establishment, the timing of flowering and the responses to neighbor competition, etc [1,2]. Photosynthetic action spectra (PAS) of plants clearly illustrate that the blue (400e500 nm) and red (600e690 nm) lights are vital parts in plant photosynthesis, phototropism and morphology formation [3,4]. Natural lighting cannot fully meet the 7e24 operation need of greenhouse industry by the rising requirements of vegetables and ornamentals. Accordingly, it is imperative to develop artificial light sources with specific performance for indoor plant cultivation [5]. Light-emitting-diodes (LEDs), as a compact and high-efficient light source, has great potential in lighting field compared with traditional incandescent or fluorescent lamps [6,7]. An ideal plant

* Corresponding author. E-mail address: [email protected] (X. Zhang). 1 Zishan Sun and Zhenpeng Zhu contributed equally to this work. https://doi.org/10.1016/j.jallcom.2019.151956 0925-8388/© 2019 Elsevier B.V. All rights reserved.

growth lighting system should have a spectrum covering three parts: blue (400e500 nm), red (620e690 nm), and far-red (700e740 nm) light, which is well-matched with the absorption spectrum of Chlorophyll a/b, Phytochrome PR and Phytochrome PFR, respectively [8,9]. By adjusting the proportion of different light, artificial control of the photosynthesis, phototropism and photomorphogenesis of the plants can be handily realized. Phosphor-converted LEDs (pc-LEDs) could be the most appropriate candidate of plant cultivation lighting, due to their mature fabrication technology and facile spectral composition control through the use of different phosphors [10]. But the re-absorption and different aging rate between each phosphor remains a problem for pc-LED using multiple phosphors [11,12]. As an alternative, single-phased multicolor-emitting phosphor could dissolve above mentioned problems, and some phosphors with distinct blue/red emission bands have been obtained using energy transfer strategy [13,14]. However, the present reported phosphors used in plant growth lighting sources suffer severe thermal quenching, which is rarely discussed and a great limitation for long-time operation of pc-LED.

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Using Ce3þ/Eu2þ as sensitizer and Mn2þ as activator, Ce3þ/Eu2þMn2þ co-doped strategy could offer both Eu2þ blue emission and Mn2þ red emission simultaneously at UV excitation, which is consistent with the photosynthetic action spectra of plants. Besides, according to previous studies [15e20], the dual-emission by Eu2þ-Mn2þ co-doped strategy is of high efficiency and thermalstable when corporated into proper hosts. Phosphates is a kind of ideal host for phosphor exploration with merits of excellent thermal stability, optical transparent and low sintering temperature [21e23]. Ca19M2(PO4)14 (M ¼ Mg, Zn, Mn) compounds crystallized in the trigonal crystal system with the R3c space group, which is isostructural with the b-Ca3(PO4)2 structure [24]. In present research, a novel NUV-excitable and dual-emitting Ca19Zn2(PO4)14: Eu2þ, Mn2þ (CZP: Eu2þ, Mn2þ) phosphor, which exhibits efficient blue and red double-band emission, is systematically studied. Highefficient (QE > 80%) and color-tunable emission are able to be adjusted by tuning Eu2þ/Mn2þ composition of CZP: Eu2þ, Mn2þ phosphors, which enables us to tailor the optimal spectrum requirements for different species of plants. Moreover, the phosphor displayed a high thermal stability at 473 K with up to 75% luminescence intensity. All results indicates that the Ca19Zn2(PO4)14: Eu2þ, Mn2þ phosphor is a promising candidate as convertedphosphor in plant growth LED lighting. 2. Experimental 2.1. Synthesis The phosphors with composition of Ca19-xZn2-y(PO4)14: xEu2þ, yMn2þ (x ¼ 0.02, y ¼ 0e2.0) are synthesized by conventional solid state reaction method. Analytically pure CaCO3, ZnO and NH4H2PO4, as well as high-purity Eu2O3 and MnCO3 were used as starting materials. Stoichiometric amounts of raw materials were weighed and ground in an agate mortar for 30 min. Subsequently, the powder mixture was transferred into corundum crucibles and calcined at 1200  C for 8 h in a tube furnace under N2eH2 reducing atmosphere. Finally, the samples were naturally cooled down to room temperature and ground into fine powder for the subsequent characterization. 2.2. Characterization X-ray diffraction (XRD) measurement was carried out by means of D/max-2000 powder diffractometer with Cu Ka radiation (l ¼ 0.154 nm). The XRD patterns were refined and the corresponding lattice parameters were obtained by MDI JADE 6.0 software. The room-temperature photoluminescence (PL) spectra were checked by utilizing a FLS-980 spectrometer set-up equipped with a 60 W Xe-arc lamp. The nanosecond lifetime was measured using a 320 nm UV laser diode attached to FLS-980 spectrometer, while microsecond lifetime was obtained using mF lamp as excitation source. The temperature dependent PL spectra of the as-prepared phosphors ranging from 293 to 473 K were detected by using FLS-980 spectrometer with a liquid nitrogen cryostat (OptistatDN). 3. Results and discussions 3.1. Structure & site-occupation The XRD patterns of Eu2þ-doped and Eu2þ/Mn2þ co-doped CZP phosphors are showed in Fig. 1a. It can be observed that all the diffraction peaks of CZP-based phosphors doped with Eu2þ/Mn2þ ions are in good agreement with the standard card of Ca19Zn2(PO4)14 (JCPDS 48e1196). The ionic radius of Ca2þ (CN ¼ 8, CN: coordination number), Ca2þ (CN ¼ 6), Zn2þ (CN ¼ 6) and P5þ

(CN ¼ 4) is 1.12 Å, 1.00 Å, 0.74 Å and 0.17 Å, respectively. Meanwhile, the ionic radius of dopant Eu2þ (CN ¼ 8), Eu2þ (CN ¼ 6) and Mn2þ (Mn ¼ 6) is 1.25 Å, 1.17 Å and 0.83 Å. Thus, It is reasonable to project that Eu2þ ions prefer to occupy 8-coordinated and 6-coordinated Ca2þ sites in the CZP host, and Mn2þ tend to locate in 6coordinated Zn2þ sites, which can be further confirmed by the XRD lattices variation. With the introduction of Mn2þ, the lattice parameters of CZP: 0.02Eu2þ, yMn2þ exhibit an expansion in comparison to the pure CZP host (Table 1), signifying the increase of lattice interplanar spacing due to relatively large ionic radius of Mn2þ. The enlarged section of XRD patterns in 30e33 are shown as Fig. 1b, where a continuous blue-shift of diffraction peak is observed with rising Mn2þ concentration. This is consistent with the lattice expansion by Mn2þ (r ¼ 0.83 Å) substitution of Zn2þ (r ¼ 0.74 Å). The crystal structure of optimal sample Ca19-0.02Zn2-0.30(PO4)14: 0.02Eu2þ, 0.30Mn2þ is solved by the Rietveld refinement of powder XRD data. The Rietveld refinement of Ca19-0.02Zn2-0.30(PO4)14: 0.02Eu2þ, 0.30Mn2þ is carried out on the basis of Ca19Cu2(PO4)14 (ICSD 50689) as an initial structural model and result shows that CZP: 0.02Eu2þ, 0.30Mn2þ crystalizes in a tetragonal cell with lattice parameters of a ¼ b ¼ 10.4123(1) Å, c ¼ 37.2925(4) Å, V ¼ 3501.42 Å3. The observed and calculated XRD patterns, as well as difference profile of the Rietveld refinement are illustrated in Fig. 2. The results of Rietveld refinement, the atomic coordinates and occupancy of CZP: 0.02Eu2þ, 0.30Mn2þ are listed in Table 2. All refined crystallographic parameters well satisfy the conditions, and good fits are obtained with Rwp ¼ 7.61%, Rp ¼ 5.61%, c2 ¼ 3.498. PXRD analysis shows a very small but systematic decrease in lattice parameter as the Mn2þ incorporation increases. The refinement result indicates that the cationic sites for Eu2þ and Mn2þ occupation is Ca1~Ca4 and Zn1 sites, respectively. XRD results indicate that the incorporation of Eu2þ and Mn2þ ions has little influence on the host crystal structure. It is reported that Ca19Zn2(PO4)14 crystallizes in the trigonal structure with the R3c (161) space group. The schematic diagram of the CZP host cell along with coordination environments of Ca2þ and Zn2þ ions are shown in the Fig. 3. There are four kinds of Ca sites and one kind of Zn site, i.e. Ca1, Ca2 and Ca3 are surrounded by eight O2 ions to form distorted dodecahedrons with different CaeO bond lengths (Ca1eO: 2.3056e2.9993 Å, Ca2eO: 2.2137e2.8223 Å, Ca3eO: 2.4016e2.6118 Å); Ca4 site is half occupied and coordinated with six O2 ions, whose bond length is ranged from 2.2415 to 2.8626 Å; Zn2þ and O2 ions constitute an irregular octahedron with ZneO bond lengths of 2.0774e2.0871 Å. As discussed above, due to the similarity of charge and ionic radius, majority of Eu2þ are located in three kinds of CaO8 sites while minority of Eu2þ are in CaO6 site and Mn2þ occupy ZnO6 site. Different occupations of Eu2þ and Mn2þ ions in the cationic sites provided various crystal environments, which would influence the photoluminescence properties. 3.2. Photo-luminescence & energy transfer The emission and excitation spectra of single Eu2þ doped CZP phosphor are given in Fig. 4a. Under the monitoring at 415 nm, the excitation spectrum shows a distinct band around 350 nm with shoulder at 300 nm, and it can be ascribed to the 4f-5d transitions of Eu2þ. The emission spectrum shows an asymmetric broad band in the range of 360 nme600 nm under 350 nm excitation, which can be well fitted two Gaussian peaks, i.e. a narrow and dominant one at 415 nm and a broad one at 530 nm. Four Ca2þ crystallographic sites exist in the crystal structure, and any type of Ca2þ may be substituted by Eu2þ ions. However, the existence of two emission peaks is inconsistent with this result. One possible explanation is the similarity of the crystal-field environments in 8-coordnated

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Fig. 1. The XRD patterns of CZP: 0.02Eu2þ, yMn2þ samples with standard card JCPDS 48e1196 (a) and the enlarged part in 30e33 (b).

Table 1 The calculated lattice parameters of CZP: 0.02Eu2þ, yMn2þ samples. Composition CZP CZP: CZP: CZP: CZP: CZP: CZP:

0.02Eu2þ 0.02Eu2þ, 0.02Eu2þ, 0.02Eu2þ, 0.02Eu2þ, 0.02Eu2þ,



0.10Mn 0.20Mn2þ 0.30Mn2þ 0.40Mn2þ 0.50Mn2þ

a (Å)

c (Å)

V (Å3)

10.3567 10.3585 10.3631 10.3748 10.3805 10.4021 10.4132

37.1730 37.2568 37.2929 37.2648 37.3284 37.3076 37.3015

3453.00 3462.08 3468.45 3473.68 3483.45 3495.98 3502.90

Fig. 2. Rietveld refinement of the powder XRD profiles of the optimal sample CZP: 0.02Eu2þ, 0.30Mn2þ.

Ca1, Ca2 and Ca3 sites, even though the exact coordination environment of these sites differs. According to Van Uitert equation, the emission wavelength of Eu2þ shifts to long wavelength with the decrease of CN value of the substituted sites [25]. Therefore, the emission band peaked at 415 and 530 nm are ascribed to the 5d-4f transitions of Eu2þ in the 8-coordinated Ca1/2/3 sites and 6coordinated Ca4 site, respectively. As reported by Wang et al. [20], irradiation light for plants' growth lies in three spectral ranges of blue (400e500 nm), red

(620e690 nm), and far red (720e740 nm), responsible for photosynthesis, phototropism, and photomorphogenesis, respectively. Among those emissions, red and blue light are dominant, while a quantity of far-red light is needed sometimes. From Fig. 4b, it is conspicuous that a double-band emission spectra, which consists of Eu2þ blue emission peak at 415 nm and sensitized Mn2þ red emission peak at 660 nm, is observed in CZP: Eu2þ, Mn2þ sample under 350 nm excitation. This blue-red double-band emission perfectly matches the requirement of irradiation light for plants’ growth. Under the monitoring at 660 nm emission, the excitation spectrum of CZP: Eu2þ, Mn2þ appears a strong and broad absorption band from 235 nm to 375 nm, which has the same shape with Eu2þ doped CZP sample and matches well with the emission band of commercial LED chip (lem ¼ ~365 nm). Both emission and excitation spectra confirms the occurrence of energy transfer from Eu2þ to Mn2þ. It is known that Mn2þ 4T1-6A1 radiative transition energy are strongly influenced by site symmetry and the crystal field strength of host. Thus, the emission of Mn2þ can be tuned from green (tetrahedral crystal field) to red (octahedral crystal field) [26]. In present case, the emission band of Mn2þ ions at 660 nm in the CZP host indicated that the Mn2þ occupy the octahedral ZnO6 site. This phenomenon agrees with the occupancy situation of the Mn2þ ions discussed previously. In order to obtain efficient dual-emitting phosphor, the emission spectra of CZP: 0.02Eu2þ, yMn2þ samples with different Mn2þ doping contents were measured. As shown in Fig. 5 the peak intensity of Mn2þ red emission continuously increase with rising Mn2þ contents, until the Mn2þ contents is above 0.30. At the same time, the peak intensity of Eu2þ blue emission decreases monotonically, which indicates that when the Eu2þ doping concentration is constant, the energy transition efficiency from Eu2þ to Mn2þ is becoming more efficient. The high quenching concentration of Mn2þ means the weak interaction among Mn2þ ions. Excitationdependent PL spectra and excitation-emission matrix of the optimal sample CZP: 0.02Eu2þ, 0.30Mn2þ with different excitation wavelength have been added as Fig. 6a and b, The emission behaviors of CZP: 0.02Eu2þ, 0.30Mn2þ were explored and showed two distinct emission peaks at 415 and 660 nm, whose peak positions were independent of the excitation wavelength, while the peak intensities reach their maximum at 350 nm excitation. The quantum efficiency with optimal composition (CZP: 0.02Eu2þ and CZP: 0.02Eu2þ, 0.30Mn2þ) is measured to be 80.6% and 93.3%

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Table 2 Crystallographic data, atomic coordinates and occupancy for Ca19-0.02Zn2-0.30(PO4)14: 0.02Eu2þ, 0.30Mn2þ Derived from Rietveld refinement of X-ray diffraction powder diffraction data. Formula

Ca19-xZn2-y(PO4)14: xEu2þ, yMn2þ (x ¼ 0.02, y ¼ 0.30)

Space group

R3c (161) - trigonal

Cell parameters

a ¼ b ¼ 10.4123(1) Å, c ¼ 37.2925(4) Å, V ¼ 3501.42 Å3, Z ¼ 6

Reliability factors

Rwp ¼ 7.61%, Rp ¼ 5.61%, c2 ¼ 3.498

Atom

Site

x

y

z

occupancy

U (Å2)

Ca1 Eu1 Ca2 Eu2 Ca3 Eu3 Ca4 Eu4 Zn1 Mn1 P1 P2 P3 O1 O2 O3 O4 O5 O6 O7 O8 O9 O10

18b 18b 18b 18b 18b 18b 18b 18b 6a 6a 6a 18b 18b 6a 18b 18b 18b 18b 18b 18b 18b 18b 18b

0.72417 0.72417 0.61624 0.61624 0.20283 0.20283 0 0 0 0 0 0.6667 0.64556 0 0.02519 0.75086 0.73457 0.72496 0.50369 0.61459 0.54373 0.79754 0.62905

0.84510 0.84510 0.82255 0.82255 0.40403 0.40403 0 0 0 0 0 0.86021 0.81355 0 0.83900 0.86272 0.75674 0.03348 0.75919 0.93340 0.69407 2.08139 0.86947

0.43423 0.43423 0.23516 0.23516 0.33991 0.33991 0.17384 0.17384 0.00060 0.00060 0.23687 0.13698 0.03506 0.31014 0.25417 0.17438 0.11714 0.12539 0.13016 0.04724 0.03787 0.03787 0.98459

0.999 0.001 0.999 0.001 0.999 0.001 0.4995 0.0005 0.85 0.15 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0

0.0392 0.0392 0.0232 0.0232 0.0382 0.0382 0.4993 0.4993 0.0629 0.0629 0.0506 0.0284 0.0047 0.0047 0.0047 0.0047 0.0047 0.0047 0.0047 0.0047 0.0047 0.0047 0.0047

Fig. 3. The schematic diagram of the CZP host cell along with coordination environments of Ca2þ and Zn2þ ions.

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Fig. 4. The emission and excitation spectra of Eu2þ-doped (a) and Eu2þ, Mn2þ co-doped CZP sample (b).

case of Ca19Zn2(PO4)14: 0.02Eu2þ, 0.30Mn2þ samples, 3 V ¼ 3483.45 Å , xc ¼ 0.30, and N ¼ 3*2 ¼ 6; thus, Rc is calculated to be 15.46 Å, which is much larger than the Rc exchange interaction mechanism (5 Å). As a result, the electric multipolar interaction energy transfer should be responsible for the concentration quenching mechanism in CZP: 0.02Eu2þ, yMn2þ. According to Refs. [28,29], the energy transfer efficiency between the sensitizer and activator can be roughly estimated from the PL emission spectra by utilizing the following expression:

h¼1 

Fig. 5. The emission spectra of CZP: 0.02Eu2þ, yMn2þ samples samples with different Mn2þ doping contents (lex ¼ 350 nm).

under 350 nm excitation. Generally, concentration quenching is triggered by the increasing interaction possibility among Mn2þ ions, which is determined by the intra-Mn2þ distance. The critical distance (Rc) at which the possibility of nonradiative energy transfer and radiative emission is equal, could be obtained as [27]:

Rc ¼ 2ð

3V 1=3 Þ 4pxc N

(1)

where V stands for unit cell volume, xc stands for the critical concentration, and N is the occupied site number in one unit cell. In

Is Is0

(2)

where the parameters of h, IS and IS0 denote the energy transfer efficiency, integrated PL emission intensities of sensitizer Eu2þ (350e550 nm) with the presence and absence of activator Mn2þ, respectively. According to Eq. (2) and the measured PL emission spectra (Fig. 5), the energy transfer efficiency as a function of Mn2þ concentration was evaluated as 20.3%, 37.7%, 53.1%, 71.6%, 77.6%, 84.5% and 89.3% for y ¼ 0.05, 0.10, 0.15, 0.20, 0.30, 0.40 and 0.50, respectively. Obviously, the estimated energy transfer efficiency was dependent on the dopant concentration and its maximum value is 89.3% at y ¼ 0.50. These results suggested that the energy transfer from Eu2þ to Mn2þ ions in CZP: 0.02Eu2þ, yMn2þ phosphors is efficient. In order to further shed light to the Eu2þ-Mn2þ energy transfer process and mechanism, the luminescent decay curves of Eu2þ in CZP: 0.02Eu2þ, yMn2þ samples are measured and shown in the Fig. 7a. The average decay lifetime (t) of Eu2þ can be calculated by the formula as followed:

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Fig. 6. Excitation-dependent PL spectra (a) and excitation-emission matrix (b) of the optimal sample CZP: 0.02Eu2þ, 0.30Mn2þ.

Fig. 7. The decay curves of Eu2þ (a) and Mn2þ emission (b) in CZP: 0.02Eu2þ, yMn2þ samples.

emission. These decay curves can be well fitted using single exponential equation:

ð

t¼ ð

tIðtÞdt (3) IðtÞdt

where I(t) is the luminescence intensity at time t. With increasing Mn2þ concentration (y), the average lifetimes of Eu2þ emission in CZP: 0.02Eu2þ, yMn2þ is shorten from 634.5 ns at y ¼ 0e57.39 ns at y ¼ 0.50. The average lifetime of Eu2þ gradually decreases along with increasing Mn2þ content, which verifies the existence of Eu2þMn2þ energy transfer. Fig. 7b depicts the Mn2þ decay curves of CZP: 0.02Eu2þ, yMn2þ samples under excitation at 320 nm and monitored at 660 nm

t IðtÞ ¼ I0 expð  Þ

t

(4)

where I0 and I(t) are the luminescence intensities at time 0 and t, respectively, and t is the fluorescence lifetime. The luminescence decay lifetimes are listed in Table 3. It can be found that the decay lifetime remains at ~45 ms when Mn2þ content is low, then starts decreasing when y is beyond 0.20, probably because of the super exchange interaction between the close Mn2þ ion pairs at high concentration, which eventually results in emission concentration quenching [30]. The dominant energy transfer mechanism between Eu2þ and

Z. Sun et al. / Journal of Alloys and Compounds 811 (2019) 151956 Table 3 The average lifetime values of Eu2þ and Mn2þ emission in CZP: 0.02Eu2þ, yMn2þ samples. Composition CZP: CZP: CZP: CZP: CZP: CZP: CZP: CZP: CZP: CZP:

0.02Eu2þ 0.02Eu2þ, 0.02Eu2þ, 0.02Eu2þ, 0.02Eu2þ, 0.02Eu2þ, 0.02Eu2þ, 0.02Eu2þ, 0.02Eu2þ, 0.02Eu2þ,

0.05Mn2þ 0.10Mn2þ 0.15Mn2þ 0.20Mn2þ 0.25Mn2þ 0.30Mn2þ 0.35Mn2þ 0.40Mn2þ 0.50Mn2þ

tEu (ns)

tMn (ns)

634.50 449.83 375.01 346.59 282.31 237.11 204.17 182.61 96.98 57.39

~ 45.97 45.68 45.33 34.37 31.88 29.81 25.25 19.37 12.92

Mn2þ can be analyzed with the Reisfeld's approximation as followed:

ts0 n=3 fC ts

7

calculated. As shown in Fig. 9a, the emitting color of the studied samples was verified from blue through violet to red with the addition of Mn2þ concentration. Meanwhile, the color coordinates of the CZP: 0.02Eu2þ, yMn2þ compounds were also changed from (0.235, 0.176) to (0.505, 0.250), as listed in Fig. 9a. These results confirmed that color-tunable emissions can be obtained in the CZP: 0.02Eu2þ, yMn2þ phosphors. The tunable color point confirms the existence of the efficient ET processes of Eu2þ-Mn2þ, whose process is depicted through the energy-level model in Fig. 9b. Upon excitation at 350 nm, electrons in Eu2þ ions can be effectively excited from 4f ground state to 5d excited states. Then the excited electrons relax to the bottom of 5d states ions and go back to the ground state of spontaneously by a radiative process. Another relaxation path is appeared when Mn2þ is introduced into CZP: Eu2þ, i.e. the excited Eu2þ ions transfer its energy to 4T2g (4G) state of Mn2þ. After relax to 4T1 (4G) state, a sensitizer Mn2þ red emission is observed from 4T1e6A1 radiative transition.

(5)

where C stands for the concentration of activator Mn2þ ions; Exchange, dipole-dipole, dipole-quadrupole, and quadrupolequadrupole interaction corresponds to n ¼ 3, 6, 8 and 10, respectively; ts and ts0 is the lifetime value of the sensitizer (Eu2þ) with and without the activator (Mn2þ). Fig. 8 gives the relationship between ts0/ts and Cn/3. A linear fitting could be only found in n ¼ 6 situation with R2 ¼ 0.9921. It suggests that the energy transfer from Eu2þ to Mn2þ in the CZP host material is dominantly by the electric dipole-dipole interaction. Based on the measured emission spectra, the Commission Internationale de I'Eclairage (CIE) coordinates of CZP: 0.02Eu2þ, yMn2þ phosphors as a function of Mn2þ concentrations were

Fig. 8. The relationship between ts0/ts and Cn/3 in CZP: 0.02Eu2þ, yMn2þ samples.

3.3. PL thermal stability Thermal stability is an important parameter that should be addressed for phosphors used in pc-LEDs. It is known that the working temperature of operated LED chip will reach to 100  C. Thus, excellent thermal stability is required for LED phosphors, which is of important to maintain high efficiency and long lifetime of the LEDs [31,32]. The temperature-dependent PL spectra of CZP: 0.02Eu2þ are shown in Fig. 10. Obviously, there is no significant change in spectral shape with changing temperature. Meanwhile, the relative emission intensity varies slightly and start a slowly declining when temperature is beyond 373 K. The emission intensity at 373 K and 473 K is 103% and 85% of its starting value at room temperature, revealing that the CZP: Eu2þ sample has admirable thermal stability and may be fulfilling candidates for developing Eu2þ, Mn2þ co-doped LED phosphors. Temperature-dependent PL spectra of optimized CZP: 0.02Eu2þ, 0.30Mn2þ phosphor are represented in Fig. 11. A normal thermal quenching behavior can be observed, where the emission intensities continuously decrease with the temperature increasing from 293 K to 473 K, due to the increased nonradiative transitions. It is found that the Mn2þ red emission at wavelengths of 550e820 nm shows a more rapid decrease than the Eu2þ blue emission ranging from 350 to 550 nm with increasing temperature. At 473 K, ~80% of the emission intensity remains for Eu2þ blue emission while 65% for Mn2þ red emission. The different thermal quenching rate could be attributed to the fact that the Mn2þ luminescence from 3d to 3d transitions has strong coupling effect with temperature variation. To be specific, the thermally activated luminescent center (Mn2þ) strongly interacts with thermally active phonons, and then thermally released through the crossing point between the excited state and the ground state in the configurational coordinate diagram. With rising temperature, the population density of phonons is increased, and the electron-phonon interaction is dominant, and consequently the thermal quenching of the emission intensity is obvious. Similar phenomenon could be found in Na3Sc2P3O12: Eu2þ, Mn2þ [33]. To further prove its potential, the thermal stability and CIE coordinates of CZP: Eu2þ, Mn2þ phosphors are compared with other reported phosphors for plant growth LEDs, as shown in Table 4. Obviously, the thermal quenching in CZP: Eu2þ, Mn2þ is smaller than most previous-reported phosphors. Meanwhile, CZP: Eu2þ, Mn2þ shows a proper CIE coordinates with dual-emission compared with those counterparts. Thus, it is reasonable to conclude that CZP: Eu2þ, Mn2þ phosphoris of great potential as converted phosphors for plant growth LEDs.

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Fig. 9. CIE diagram (a) and energy transfer scheme (b) in CZP: Eu2þ, Mn2þ system.

Fig. 10. The temperature-dependent emission spectra of CZP: 0.02Eu2þ sample (lex ¼ 350 nm, inset is the normalized integrated emission area with changing temperature).

Fig. 11. The temperature-dependent emission spectra of CZP: 0.02Eu2þ sample (lex ¼ 350 nm), inset is Eu2þ (360e550 nm) and Mn2þ (550e800 nm) normalized integrated emission area with changing temperature.

Table 4 The comparison of thermal stability and CIE coordinates of CZP: Eu2þ, Mn2þ along with other reported phosphors for plant growth LEDs. Composition

CIE coordinates (x, y)

Thermal stability (PL intensity at 100  C vs RT value)

Na3La(PO4)[email protected]: Eu [5] Ba3CaK(PO4)3: Eu2þ [9] CsPb0.7Ti0.3I3 [email protected] [34] Sr2SiO4: Eu2þ-Ba3MgSi2O8: Eu2þ, Mn2þ [35] ZnGa2O4: Cr3þ PiG [36] La(MgTi)1/2O3: Mn4þ [37] CDs/CaAlSiN3:Eu2þ-silica [38] Ca19Zn2(PO4)14: Eu2þ, Mn2þ

(0.258, (0.243, (0.355, (0.385,

60% 62% N/A N/A

0.106) 0.287) 0.376) 0.344)

(0.668, 0.255) N/A (0.293, 0.244) (0.464, 0.238)

85% 71.4% N/A 83.6%

Z. Sun et al. / Journal of Alloys and Compounds 811 (2019) 151956

4. Conclusion In summary, a series of novel dual-emitting Ca19Zn2(PO4)14: Eu2þ, Mn2þ phosphors had been synthesized by a solid-state method. The phosphor exhibits a broad UV absorption band and an emission spectra consisting of blue and red bands, which matches well to the absorption spectra of Chlorophyll a/b, Phytochrome PR and Phytochrome PFR in plants. Either intensity ratio of blue to red band and emitting color can be tailored by tuning Eu2þ/ Mn2þ. Eu2þ-Mn2þ energy transfer in the phosphors was investigated on the bases of emission spectra and decay time variations, and the corresponding mechanism is demonstrated to be a resonant type via the dipole-dipole interaction. The quantum efficiency with optimal composition (CZP: 0.02Eu2þ and CZP: 0.02Eu2þ, 0.30Mn2þ) is measured to be 80.6% and 93.3% under 350 nm excitation. Thermal stability of CZP: Eu2þ, Mn2þ phosphors are also investigated, which show the intensity losses of ~20% for the blue emission and ~35% for the red emission at 473 K. This work suggests that CZP: Eu2þ, Mn2þ phosphors hold great potential for applications in plant cultivation LEDs. Acknowledgement The work was supported by National Natural Science Foundation of China (No. 21601081 and 51472132) and Guangzhou Scientific planning program (No. 201804010260). On the eve of the wedding of Zishan Sun and her bridegroom Yiwu Wu, all authors express their best wishes to this young couple for a lifetime of happiness. Congratulations! References [1] A. Zhang, M. Jia, Z. Sun, G. Liu, Z. Fu, T. Sheng, P. Li, F. Lin, High concentration Eu3þ-doped NaYb(MoO4)2 multifunctional material: thermometer and plant growth lamp matching phytochrome P-R, J. Alloy. Comp. 782 (2019) 203e208. [2] L. Shi, Y. Han, H. Wang, D. Shi, X. Geng, Z. Zhang, High-efficiency and thermally stable far-red emission of Mn4þ in double cubic perovskite Sr9Y2W4O24 for plant cultivation, J. Lumin. 208 (2019) 307e312. [3] L. Shi, Y. Han, Z. Zhang, Z. Ji, D. Shi, X. Geng, H. Zhang, M. Li, Z. Zhang, Synthesis and photoluminescence properties of novel Ca2LaSbO6: Mn4þ double perovskite phosphor for plant growth LEDs, Ceram. Int. 45 (2019) 4739e4746. [4] J. Hu, T. Huang, Y. Zhang, B. Lu, H. Ye, B. Chen, H. Xia, C. Ji, Enhanced deep-red emission from Mn4þ/Mg2þ co-doped CaGdAlO4 phosphors for plant cultivation, Dalton Trans. 48 (2019) 2455e2466. [5] M. Xia, X. Wu, Z. Zhou, W. Wong, A novel Na3La(PO4)2/LaPO4:Eu blue-red dual-emitting phosphor with high thermal stability for plant growth lighting, J. Mater. Chem. C. 7 (2019) 2385e2393. [6] S. Liao, X. Ji, Y. Liu, J. Zhang, Highly efficient and thermally stable blue-green (Ba0.8Eu0.2O)(Al2O3)(4.575x(1þx)) phosphor through structural modification, ACS Appl. Mater. Interfaces 10 (2018) 39064e39073. [7] X. Ji, J. Zhang, Y. Li, S. Liao, X. Zhang, Z. Yang, Z. Wang, X. Qiu, W. Zhou, L. Yu, S. Liao, Improving quantum efficiency and thermal stability in blue-emitting Ba2-xSrxSiO4: Ce3þ phosphor via solid solution, Chem. Mater. 30 (2018) 5137e5147. [8] J. Chen, C. Yang, Y. Chen, J. He, Z.Q. Liu, J. Wang, J. Zhang, Local structure modulation induced highly efficient far-red luminescence of La1-xLuxAlO3: Mn4þ for plant cultivation, Inorg. Chem. 58 (2019) 8379e8387. [9] J. Xiang, J. Zheng, Z. Zhou, H. Suo, Q. Zhao, X. Zhou, N. Zhang, M. Molokeev, C. Guo, Enhancement of red emission and site analysis in Eu2þ doped newtype structure Ba3CaK(PO4)3 for plant growth white LEDs, Chem. Eng. J. 356 (2019) 236e244. [10] Q. Zhou, L. Dolgov, A. Srivastava, L. Zhou, Z. Wang, J. Shi, M. Dramicanin, M. Brik, M. Wu, Mn2þ and Mn4þ red phosphors: synthesis, luminescence and applications in WLEDs. A review, J. Mater. Chem. C. 6 (2018) 2652e2671. [11] W. Liu, X. Wang, J. Li, Q. Zhu, X. Li, X. Sun, Gel-combustion assisted synthesis of eulytite-type Sr3Y(PO4)3 as a single host for narrow-band Eu3þ and broadband Eu2þ emissions, Ceram. Int. 43 (2017) 15107e15114. [12] F. Ruan, D. Deng, M. Wu, C. Wu, S. Xu, Tunable single-host full-color-emitting Ca9Zn1.5(PO4)7: Eu, Tb phosphor via Eu2þ/Eu3þ dual-emitting, J. Lumin. 198 (2018) 1e9. [13] X. Zhang, Y. Huang, M. Gong, Dual-emitting Ce3þ, Tb3þ co-doped LaOBr phosphor luminescence, energy transfer and ratiometric temperature sensing, Chem. Eng. J. 307 (2017) 291e299. [14] X. Zhang, J. Zhang, Y. Chen, Broadband-excited and efficient blue/green/red-

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