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Plant habitat-conscious phosphors: Tuneable luminescence properties of Dy3+-doped Ca8ZnY(PO4)7 phosphors by co-dopants Mg2+ and B3+
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Plant habitat-conscious phosphors: Tuneable luminescence properties of Dy3+-doped Ca8ZnY(PO4)7 phosphors by co-dopants Mg2+ and B3+ Yongli Zhanga,b,1, Minghui Lia,1, Zihui Konga, Chao Liangd, Cheng Zhoua, Mao Xiaa,b,c,∗∗, Zhi Zhoua,b,∗ a
College of Chemistry and Materials Science, Hunan Agricultural University, Changsha, 410128, PR China Hunan Optical Agriculture Engineering Technology Research Center, Hunan Province, 410128, PR China c State Key Laboratory of Powder Metallurgy, Central South University, Changsha, Hunan, 410083, PR China d Jiangsu Bree Optronics Co., Ltd., Nanjing, 211103, PR China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Devices Habitat-conscious Phosphor Dy3+-activator
Outdoor lighting and other lighting systems can disrupt natural plant growth habits. Thus, LED lighting that is not detrimental to plant growth is required. In our study, Dy3+-doped Ca8ZnY(PO4)7:Dy3+ phosphor with enhanced luminescence properties caused by the co-dopants Mg2+ and B3+ were synthesised. The samples had multiple excitation peaks, indicating they are excited by either near-ultraviolet (n-UV) or blue chips. All samples exhibited bright narrow yellow and blue emission corresponding to the transitions of Dy3+ ions with 4F9/ 6 4 6 2+ and B3+ enhanced the luminescence 2→ H13/2 and F9/2→ H13/2, respectively. Moreover, doping with Mg intensity, reaching 113.6 and 119.7%, respectively. In addition, the luminescence emission intensity at 150 °C was maintained at approximately 95% of the initial value at 25 °C, and its thermal stability increased by 123%. Devices assembled with an n-UV chip (388 nm) and the as-obtained CZMYP:Dy3+ phosphor emitted a bright warm white light and simulated outdoor dark lighting for tobacco cultivation, indicating that the as-prepared phosphor is an excellent candidate material for plant habitat-conscious phosphors.
1. Introduction Light is the source of energy required for plant growth and an important environmental signal regulating photosynthesis, material metabolism and gene expression [1–3]. The study of supplementary plant light is of great significance to economic development [4]. Compared with the ordinary fluorescent lamp, LED lamp have evident advantages: no mercury, simple structure, good seismic performance, high light efficiency, pure light quality, low energy consumption and other photoelectric advantages [5,6]. LED is recognised as the most promising electric light source in the 21st century, and it is widely used in the research of cultivated crops. The combination of the blue InGaN LED, yellow-emitting phosphor (Y3Al5O12:Ce3+), and red-emitting nitride phosphor is the most frequently used method to package LEDs [7]. However, the preparation conditions for these nitride phosphors are harsh, as they often require a high temperature (> 1800 °C) and high pressure in an oxygen-free environment [8–10]. Moreover, The combination of the blue InGaN LED chip and the two types of phosphor can
lead to waste of materials or uneven mixing, causing a reduction in the quantum efficiencies of samples [4]. A large Stokes shift and the intense reabsorption of blue light by red phosphors are the possible reasons. However, these problems are easily solved by the single-phase phosphor because of higher luminous efficiency, small colour aberration, excellent colour rendering index (CRI), and low cost. Most researchers have focused on white LEDs (WLEDs) that associate with people's colour perception. However, the fluorescent lamps that emit light brighter light have negative effects on the natural environment [11]. At night, the prolonged exposure of ordinary WLED lights can inhibit the flowering and fruiting of short-day plants and promote the flowering and fruiting of long-day plants, especially in caves and forests and roadsides where many lights are used [12]. Finally, the growth of plants is affected, natural plant habitats are destroyed, and crop yields decrease. Phosphor converted LEDs, as the primary artificial light source, are fabricated by combining various chips and matched phosphors [13]. The luminescence properties of assynthesised phosphor materials are significant parameters LED lighting
∗
Corresponding author. College of Chemistry and Materials Science, Hunan Agricultural University, Changsha, 410128, PR China. Corresponding author. College of Chemistry and Materials Science, Hunan Agricultural University, Changsha, 410128, PR China. E-mail addresses: [email protected] (M. Xia), [email protected] (Z. Zhou). 1 Yongli Zhang and Minghui Li contributed equally to this work. ∗∗
https://doi.org/10.1016/j.ceramint.2020.01.203 Received 24 December 2019; Received in revised form 19 January 2020; Accepted 21 January 2020 0272-8842/ © 2020 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Yongli Zhang, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2020.01.203
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thermal stability of the phosphor were studied detail. Furthermore, the corresponding reasons and enhancement mechanism are investigated.
applications for plant growth. Thus, changing the impact of outdoor lighting on local plant local species by changing the properties of phosphors is necessary. Dysprosium is considered a familiar activator with abundant energy levels, high colour purity, and high conversion and luminescence efficiency for the design of suitable phosphors [14]. For example, Nakajima studied the effects of WLEDs made of phosphors doped with Dy3+ on the photosynthesis of chlorella and observed that these lamps inhibited the intensity of photosynthesis to 26% due to the reduction of red-nearIR emissions [11]. This also proves that compared with ordinary LED lights, these WLEDs can indeed reduce the photosynthesis of plants at night. The luminescent materials of phosphate hosts have the advantages of structural diversity, excellent thermal stability and steadiness of ionic charge in the lattice [15]. Many reports indicate that (β-Ca3(PO4)2)-type as a potential whitlockite-type crystal texture compounds have five types of Ca sites, namely, Ca1–Ca5 [16,17]. All except Ca4 are completely filled, and the Ca4 site is only half occupied. We can assume a crystal structure whose position is completely filled or completely empty, because its stable structure is related to the half-occupied Ca4 [18–20]. This particular crystal texture allows abundant heterovalent substitution by forming vacancies. In general, Ca2+ can be replaced with a univalent ion M+ (Li or Na), a bivalent ion M2+ (Mg or Zn), and a trivalent ion R3+(R = Y, Ga, In) in the structure of β-Ca3(PO4)2 to obtain different compounds [18]. Teterskii et al. researched the properties of Ca9-xMxR (PO4)7(M = Mg, Zn; R = Ln, Y; 0 < x < 1.5) which is also a new family of whitlockite-type hosts [21]. Additionally, Jang et al. studied the luminescence of Ca8MgR(PO4)7:Eu3+ (R = La, Gd, Y) on the basis of Teterskii's study [19]. We have recently demonstrated the great thermal stability of a novel Ca8ZnY(PO4)7:Dy3+ phosphor. However, although it was successfully synthesised, the luminescent property was still not sufficiently high to meet the demand for high-power lighting applications. Therefore, improving the phosphors property by various methods is still necessary. The luminescence properties of as-synthesised phosphor materials are significant parameters for plant-growth LED lighting applications [22,23]. Enhancing luminescence properties is mainly achieved through the following three methods: 1) charge compensation. This can modify the composition of defects to construct an optimal activator environment or alter the local site symmetry of the host structure by introducing a foreign ion [24,25]. For instance, Peng et al. reported that charge compensator ions (Li+, Na+, and K+) in SrAl2O4:Eu3+ phosphor enhance its luminescence properties [26]; 2) adding a fluxing agent. This can decrease the probability of non-radiation transition and sintering temperature for energy conservation [27,28]. The emission intensity and quantum efficiency of Lu3-xAl5O12:xCe3+ phosphor can be enhanced by adding various fluxing agent, and BaF2 (3 wt%) exhibited the best effect [29]; 3) the solid solution effect. This plays a key role in the design and preparation of Dy3+-doped Ca8(Zn/Mg)Y(PO4)7 phosphors. The solid solution effect mainly replaces the lattice position of the host ion by introducing a foreign ion, causing enhanced thermal stability and quantum efficiencies of samples via changing the crystal local structure of the matrix [30–32]. For instance, Qiao et al. reported a red emission (Ba1−xSrx)2YSbO6:Mn4+ phosphor through tuning of the compositions and multiple activator sites, and the thermal stability increased significantly via Sr2+ replacing Ba2+ [33]. Analogously, the thermal stability of Ca3−xSrx(PO4)2:Eu2+ phosphor is improved based on the solid solutions method [34]. Therefore, we assumed Mg2+ replaced Zn2+ in Ca8ZnY(PO4)7:Dy3+ phosphor to enhance its thermal stability and luminescence properties. In this work, Ca8ZnY(PO4)7:Dy3+, Ca8Zn1–yMgyY(PO4)7:Dy3+ and boron-ion-co-doped Ca8Zn1–yMgyY(PO4)7:Dy3+, z%wtB3+ phosphor were prepared through the high-temperature approach. First, the luminescence behaviour and mechanism of Dy3+-activated Ca8ZnY(PO4)7 phosphor are discussed. In addition, the effect of doping Mg2+ ion and co-doping with B3+ ion on the photoluminescence (PL) properties and
2. Experimental sections 2.1. Materials Ca8ZnY1-x(PO4)7:xDy3+ (CZYP:Dy3+; x = 0.06, 0.09, 0.12, 0.15, and 0.18, molar ratio) and Ca8(Zn1–yMgy)Y(PO4)7:Dy3+ (CZMYP:Dy3+; y = 0, 0.2, 0.4, 0.6, 0.8, and 1.0 M ratio) phosphors were compounded using the traditional high temperature method. The original reagents were CaCO3 (AR), Dy2O3 (99.99%), NH4H2PO4 (99.99%), MgO (99.99%), Y2O3 (99.99%) and ZnO (99.99%) bought from Aladdin Reagent (Shanghai) Co., Ltd, and different contents of H3BO3 (AR) acted as the flux in the reaction process to obtain the ideal samples. All initial materials were weighted proportionately and the mixtures were thoroughly ground in corundum crucibles for 30 min. Subsequently, at the heating rate of 5 °C/min, powders were heated up to 1300 °C for 5 h in the ambient atmosphere. After sintering, the furnace was closed to let the samples to cool to the ambient temperature. Finally, the samples were removed and carefully pulverised into fine powders for further analysis and characterisation. 2.2. Measurement The X-ray powder diffraction (XRD) patterns ranging from 10 to 80° were obtain using a diffractometer (D/Max-2200/pc, Rigaku); the sealed Cu-Kα X-ray tube was set at 40 kV and 20 mA. The elemental mapping images and compositional elements analysis of the CZYP:0.12Dy3+ phosphor were recorded using an energy dispersive Xray detector (EDX). The UV–vis absorption spectra were recorded on the U-3310 spectrophotometer (Hitachi, Japan). An F-4700 fluorescence spectrophotometer (Hitachi, Japan) equipped with an excitation light source (150W Xe lamp) measured the photoluminescence excitation (PLE), PL spectra and PL spectra with temperature variation. The decay times for the prepared phosphors were measured on an FLS920 spectrometer (Edinburgh, UK). The phosphors were coated on blue chips with a 470 nm emission. The electro-luminescence (EL) spectra and parameters of the pc-LED devices were measured by a LED spectrophotocolorimeter (ATA-500, Everfine). 3. Results and discussion 3.1. Crystal structure and purity analysis The XRD patterns of the Dy3+-doped CZYP phosphors are illustrated in Fig. 1b. The results of one study indicate that Ca9-xMxR(PO4)7 (x = 1–1.5; M = Mg, Zn, and Cd; R = Ln and Y) is iso-structural to Ca9Ln(PO4) [21]. The phase purity of the phosphor was analysed by comparing them with the standard card of PDF#49–0503; the as-prepared phosphors indicated they were almost crystallised in a purity phase with no structural changes or impurity peaks. When a small quantity of Dy3+ replaced Y3+ in the CZYP matrix, the XRD patterns maintained the original position without any shift. However, the XRD patterns indicated a move to a higher angle when Mg2+ replaced Zn2+; the experimental results are shown in Fig. S1. Typically, the CZYP phosphor has a the hexagonal structure with an R3c(161) space group [35] (in Fig. 1a). The structure parameters of the compound were a = 10.40080 Å, b = 10.40080 Å, c = 37.27200 Å, V = 3491.7793 Å3, and Z = 6. Ca was observed to be surrounded by O atoms and [PO4] tetrahedrons (in Fig. 1c). Three coordination environments surrounded the O atoms by 8, 8, and 9, respectively. Fig. 2 depicts the elemental mapping images of the CZYP:Dy3+ sample. The figure distinctly shows that the components of Ca, O, Y, Dy, P, and Zn were distributed in the target particle evenly, indicating that Dy successfully into the crystalline structure of the CZYP host. In 2
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Fig. 1. (a) XRD pattern of as-prepared CZYP:Dy3+ phosphors; (b) The crystal structure of CZYP compound.
concentration quenching effect. Blasse pointed out that the non-radiative energy transfer in the process of luminescence of oxalic phosphors is due to the resonance transfer of electric multipoles or exchange interaction. Additionally, the mechanism is determined by calculating the critical distance (RC) between the neighbouring Dy3+ ions. There are two scenarios: when the distance of Dy–Dy is less than 5 Å, the exchange interaction is effective, and when the Dy–Dy distance is greater than 5 Å, the most interaction is multipolar. According to Blasse, RC can be calculated through the following formula [2,5,36,37]:
addition, the calculated atomic ratio was approximate to the desired formula, indicating the successful synthesis of the target phosphor. 3.2. Luminescence properties of CZYP:xDy3+ Fig. 3a depicts the UV–vis absorption spectrum of CZYP, CZYP:0.12Dy3+ and CMZYP:0.12Dy3+ samples. The figure shows that all specimens exhibited narrow absorption bands in the 200–250 nm wavelength range, which were caused by the host absorption of CZYP. In the wavelength range of 250–350nm, the absorption intensity of CZYP and CMZYP:0.12Dy3+ tended to decline compared with the previous band. This can be clearly understood because the Dy3+-doping enhanced the absorption intensity. In addition, the peak positions of excitation spectrum match with the absorption spectrum. In general, CZYP:0.12Dy3+ phosphors have a strong absorption capacity, indicating that the addition of Dy3+ is beneficial to the photoluminescence of CZYP:0.12Dy3+. To better understand the band structure of the CZYP:0.12Dy3+ sample, we calculated the band gap energies of CZYP, CZYP:0.12Dy3+ and CMZYP:0.12Dy3+ samples using the following formula [4,31]:
(αhν )2 = A (hν − Eg )
1
3V ⎤ Rc ≈ 2 ⎡ ⎢ 4πχ c N ⎥ ⎣ ⎦
3
(2) 3+
In this formula, Xc represents the critical concentration of Dy , V represents unit cell volume, and N is the number of dopant effective sites per cell. In this study, N was 6, V is 3491.7793 Å3, and Xc was 0.12. Finally, the RC of the CZYP:Dy3+ is determined as approximately 21 Å. Moreover, the value of θ represents dipole-dipole interactions (θ = 6), dipole-quadruple interactions (θ = 8) and quadruple-quadruple interactions (θ = 10), respectively. In general, the three types can be determined by the value of Rc, such as the exchange interaction (Rc < 5 Å). On the basis of Van Uiter's theory, three forms of energy transfer occur in multipolar interactions. The form of multipolar interaction can be decided according to the intensity variation of the emission level with multipolar interactions. The relationship between an activator ion (x) and emission intensity (I) is as follows [1,38,39]:
(1)
where h stand for the Planck constant, α is the absorption coefficient, v is the frequency constant, and Eg represents the band gap energy. As shown in Fig. 3b, the specific values of the band gap energies of CZYP, CZYP:0.12Dy3+, and CMZYP:0.12Dy3+ were determined to be 4.63, 4.28, and 4.54 eV, respectively. As the figure shows, the energy gap of the CZYP host with Dy3+ doping decreased from 4.63 to 4.28 eV, which was beneficial to the luminescence properties of CZYP:Dy3+ and exhibited excellent thermal stability through reducing the possibility of photoionisation. The excitation and emission spectra of CZYP:xDy3+ (x = 0.06, 0.09, 0.12, 0.15, and 0.18) are investigated as displayed in Fig. 4. The effect of diverse doping concentrations of Dy3+ on PL intensity is shown in Fig. 4a. As the figure shows, when the critical concentration of Dy3+ was 0.12, the emission intensity reached its maximum value. The PL intensity decreased with increase in Dy3+ content because of the
I / x = K [1 + β (x )Q/3]−1
(3)
Here, I represent the PL intensity, x represents the concentration of Dy3+ ions, β and K represents constants of the CZYP structure, respectively. Q is a specific constant whose different values reflect the energy transfer patterns of multipolar interaction (6 for d-d, 8 for d-q, and 10 for q-q). Fig. 4c shows the linear relationship between log (I/x) and log (x), and the slope of the fitting line was –1.069. Finally, Q was calculated as approximately 3.207, indicating that the main mechanism of Dy3+ ions concentration quenching was a dipole-dipole interaction. At the same time, excitation spectra of CZYP:0.12Dy3+ were 3
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Fig. 2. (a)–(g) The elemental mapping images of total elements, O, Ca, Y, P, Dy and Zn, respectively; (h) The corresponding energy-dispersive X-ray spectroscopy (EDS) measurement of CZYP:0.12 Dy3+ phosphor.
monitored as shown in Fig. 4b. The figure shows some main peaks located at 324,340, 350, 362, 388, 425,450, and 473 nm and were assigned to the transitions from 6H15/2→4M17/2, 4I9/2, 6P7/2, 6P5/2, 4I13/2, 4 G11/2, 4I15/2 and 6F9/2, respectively, because of the specific electronic configuration of Dy3+. To better understand the influence of Dy3+ concentration on luminescence performance, we selected samples with doping concentrations of 0.6 and 0.12 and their decay curves were tested under the conditions of 288-nm excitation and 574 nm monitoring. Fig. 4d shows the Ca8ZnY1-x(PO4)7:xDy3+ (x = 0.6, x = 0.12) phosphors PL decay curves. Moreover, the decay data of specimens can be fitted well using the model of single exponential decay, which is shown below [13,40,41]:
−t I = I0 × exp ⎛ ⎞ ⎝ τ ⎠
killer locus increased. As a result, the lifetimes decreased with increasing Dy3+ concentrations. The luminescence schematic diagram of the Dy3+-doped CZYP substrate is shown in Fig. 5. First, many electrons were excited under ultraviolet light and some holes were formed. Because the large radiant energy was greater than the band gap energy, an electron could jump immediately from the valence band to the conduction band. The high energy level original electron group relaxed to 4F9/2 by the process of non-radiative relaxation (NR). Eventually, all the electrons could be radiated back to the ground state. Two distinct centres of light corresponded to the two types of transitions of the magnetic dipole (4F9/2 →6H15/2) and electric dipole (4F9/2→6H13/2). The magnetic dipole of the 4F9/2 to 6H15/2 transition (blue emission) is known to dominate when Dy3+ ions are in highly symmetric positions with a centre of inversion. Moreover, when Dy3+ ions are in low symmetric positions with no inversion centres, the electric dipole (4F9/2→6H13/2) transition (yellow emission) is dominant. In the experiment, the PL intensity at 574 nm (yellow emission) was stronger than the 484 nm blue emission, indicating that Dy3+ ions in this model occupied a low-symmetry site.
(4)
where τ and t represent luminescence lifetimes and time, and I and I0 stand for the luminescence intensities at t and 0. When the doping concentrations of Dy3+ were 0.06 and 0.12, the lifetimes were 0.512 and 0.425 ms, respectively. As the concentration of the Dy3+ increased, the distance of Dy―Dy decreased. Therefore, the rate of energy transfer between Dy―Dy and the energy transfer possibility to the luminous 4
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3.3. Enhanced luminescence of CZYP:0.12 Dy3+ phosphor through doping Mg2+ and B3+ Here, tuning of the emission intensity through co-doped Mg2+ and B3+ ions is discussed to further improve the luminescence intensity of the samples, as shown in Fig. 6. This can be seen in the illustration in Fig. 6a. The PL intensity of is maximum when the Mg2+ ions doping content is 0.6 and the photoluminescence intensity of phosphor increased by 113.6%. Moreover, adding a H3BO3 fluxing agent could enhance the photoluminescence intensity of the phosphors. In Fig. 6b, when the B3+ ions doping amount was 1.0%, the emission intensity was the highest. The photoluminescence intensity was detected with a 119.7% improvement in this case. The results indicated that the luminescence performance of CZYP:0.12Dy3+ phosphors could be significantly improved by co-doping Mg2+ and B3+ into the crystal structure. The two substances can enhance the luminescence intensity because they can alter the crystal environment of activator ions, promote the activator ions to enter the matrix lattice, control the size of phosphor particles and enhance the luminescence intensity.
3.4. Thermal stability of CZYP:0.12 Dy3+ and CZMYP:0.12 Dy3+ phosphors Thermal stability is known to be a significant index in checking if phosphors can be used in practice because LEDs operate at a higher working temperature than 25 °C. Fig. 7a shows the 3D temperaturedependent PL spectra of the CZYP:0.12Dy3+ phosphors, with the main emission peaks located at approximately 484 and 574nm, due to energy level transition (4F9/2→6H15/2, and 6H13/2). We can clearly see that like for most phosphors, the luminescence intensity of CZYP decreased with increasing temperature, which was related to the thermal quenching. However, based on the previous research of enhancing luminescence, we observed an interesting phenomenon. When the Mg2+ ion doping content was 0.6, the thermal stability of phosphor at 150 °C was 95%, an 8% increase from that of the un-doped phosphor. High thermal stability also provides the premise for the application of high-power LED. Moreover, activation energy (Ea) is widely used to test thermal
Fig. 3. (a) The UV–vis absorption spectra of CZYP and CZYP:Dy3+. (b) The band gap calculated based on above absorption data.
Fig. 4. (a) The concentration-dependent emission spectra of CZYP:xDy3+ (x = 0.06, 0.09, 0.12, 0.15, 0.18); (b) the excitation spectra of CZYP:0.12 Dy3+; (c) The plot of Log(I/x) as a function of Log(x); (d)decay curves of CZYP:xDy3+ phosphors. 5
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Fig. 5. Emission spectra of different contents of (a–d) H3BO3, Li+, Na+ and K+ (n = 0–1.5, x = 0–1.5, y = 0–1.5, z = 0–1.5) co-doping CSYAO:1.2%Mn4+ phosphors.
Fig. 6. Energy level and electron transitions schematic diagram of Dy3+-doped CZYP host.
Fig. 7. (a) 3D temperature-dependent PL spectra of CZYP:0.12Dy3+ phosphors; (b) The emission spectra of CZYP:0.12Dy3+ and vurse temperature CMZYP:0.12Dy3+ (25–200 °C); (c) The relationship between emission intensity and temperature of phosphors; (d) Arrhenius fitting of emission and intensity of CZYP: 0.12Dy3+ 3+ CMZYP:0.12 Dy .
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Fig. 8. The emission spectra of CZYP:0.12Dy3+ (a) and CMZYP:0.12Dy3+ (b) vurse temperature (25–200 °C). (c) The coordinate diagram of excited and ground states of Dy3+.
resistance of phosphors. For the same phosphors, a higher Ea value signifies stronger thermal stability. The following formula is used to compute Ea [42,43]:
ln[(I0/ I ) − 1] = −Ea/kT + c
(5)
Here, I represent the emission intensity at T and I0 is the PL intensity at indoor temperature. C and T denote a constant and Kelvin temperature, respectively. And k represents Boltzmann constant (k = 8.62 × 10−5 eV) Fig. 7c shows the linear relation between I/kT and In (I0/I–1). The slope of CZYP:0.12 Dy3+ was – 0.322, and Ea was determined to be 0.322eV. The slope of CMZYP:0.12 Dy3+ was – 0.381, and Ea was determined to be 0.381eV, which was larger than that of CZYP. Therefor adding 0.6 Mg improves the thermal stability of the samples. In addition, we created a comparison chart. Fig. 8a and b show the comparison of emission spectra of two specimens with the change in temperature. The figure clearly shows that, compared with CZYP host, the decreasing tendency in CMZYP with temperature change is particularly slow, and the emission intensity of Dy3+ is still very high even at 200 °C. In Fig. 8c, the different curves for 4I13/2—4I15/2—4F9/2 and 6 H9/2—6H11/2—6H13/2—6H15/2 represent the excited and ground states of Dy3+, respectively. A is the bottommost dot at 4F9/2. The curves representing the excited states and ground states intersect at point M. Under UV excitation at 388 or 452nm, electrons in the energy level (6H9/2, 6H11/2, 6H13/2 and 6H15/2) are first excited into the excited states at 25 °C. Additionally, through radiative transition, many electrons in 4 I13/2, 4I15/2, or 4F9/2 moved from point A to the ground states. As the temperature increased, the electronically excited state may have conquered the activation energy(ΔE1)because of strong phonon-electron coupling and straight into tunnel D. Therefore, ΔE1 is related to the thermal quenching. 3.5. Optical characteristics and LED manufacturing To confirm the potential application of phosphors, we tested the chromaticity coordinates and LED lamp of CZYP:0.12Dy3+, as shown in Fig. 9a. A standard single phase white luminescent phosphor that is suitable for outdoor lighting is realised. The electroluminescence spectra of the white LED lamps based on a 388nm blue-chip and CZYP:Dy3+ phosphor and driven with a current of 150 mA are shown in Fig. 9b. The LED device including CZYP:Dy3+ phosphor exhibited a stronger yellow and blue emission in the range of 484, 574nm. As expected, the emission region does not match with the plant pigments
Fig. 9. (a) CIE chromaticity coordinate of CZYP:Dy3+. (b)The electro-luminescent (EL) spectra of as-fabricated pc-LED device with the CZYP:Dy3+ phosphor and the absorption curves of Chlorophyll B and phytochrome PFR. The inset shows the appearance of as-packaged LED apparatus and the photograph in operation at current of 150 mA.
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absorption, and the phosphor can be used to effectively inhibit plant photosynthesis and protect natural plant habitats.
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Rusakov, Whitlockite solid solutions Ca9−xMxR(PO4)7 (x = 1, 1.5; M = Mg, Zn, Cd; R = Ln, Y) with antiferroelectric properties, Russ. J. Inorg. Chem. 52 (2007) 308–314. [22] Z. Xia, J. Zhou, Z. Mao, Near UV-pumped green-emitting Na3(Y,Sc)Si3O9:Eu2+ phosphor for white-emitting diodes, J. Mater. Chem. C 1 (2013) 5917–5924. [23] F. Li, Y. Liu, H. Wang, Q. Zhan, Q. Liu, Z. Xia, Postsynthetic surface trap removal of CsPbX3 (X = Cl, Br, or I) quantum dots via a ZnX2/hexane solution toward an enhanced luminescence quantum yield, Chem. Mater. 30 (2018) 8546–8554. [24] M.H. Fang, S. Mahlik, A. Lazarowska, M. Grinberg, M.S. Molokeev, H.S. Sheu, J.F. Lee, R.S. Liu, Structural evolution and effect of the neighboring cation on the photoluminescence of Sr(LiAl3)1-x(SiMg3)xN4:Eu2+ phosphors, Angew. Chem. Int. Ed. 58 (2019) 7767–7772. [25] X. Ji, J. Zhang, Y. Li, S. Liao, X. Zhang, Z. Yang, Z. Wang, Z. Qiu, W. Zhou, L. Yu, S. Lian, Improving quantum efficiency and thermal stability in blue-emitting Ba2–xSrxSiO4:Ce3+ phosphor via solid solution, Chem. Mater. 30 (2018) 5137–5147. [26] I.P. Sahu, Enhance luminescence by introducing alkali metal ions (R+ = Li+, Na+ and K+) in SrAl2O4:Eu3+ phosphor by solid-state reaction method, Radiat. Eff. Defect Solid 171 (2016) 511–527. [27] J. Long, Y. Wang, R. Ma, C. Ma, X. Yuan, Z. Wen, M. Du, Y. Cao, Enhanced luminescence performances of tunable Lu3-xYxAl5O12:Mn4+ red phosphor by ions of Rn+ (Li+, Na+, Ca2+, Mg2+, Sr2+, Sc3+), Inorg. Chem. 56 (2017) 3269–3275. [28] S. Xu, L. Sun, Y. Zhang, H. Ju, S. Zhao, D. Deng, H. Wang, B. Wang, Effect of fluxes on structure and luminescence properties of Y3Al5O12:Ce3+ phosphors, J. Rare Earths 27 (2009) 327–329. [29] Y. Yu, H. Wang, L. Li, Y. Chen, R. Zeng, Effects of various fluxes on the morphology and optical properties of Lu3−xAl5O12:xCe3+ green phosphors, Ceram. Int. 40 (2014) 14171–14175. [30] C. Liu, Y. Wang, Y. Hu, R. 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4. Conclusion A high stability single-phase white emission CZYP:Dy3+ phosphor was prepared generally using the high-temperature method. The phosphors had a rich electron transitions because Dy3+ has a special 4f orbital. Excited at 388nm, two main emission spectra, 484 and 574nm, were detected corresponding to the two types of transition (4F9/ 6 6 2→ H15/2, and H13/2), respectively. Because dipole-dipole interaction 3+ of Dy caused concentration quenching, thus the best doping of Dy3+ was 0.12 and the CIE chromaticity coordinates were (0.3326, 0.3862). According to the test, the luminous intensity of the sample could retain 87% of the room temperature when warming up to 150 °C, and the thermal stability of phosphor increased to 95% with Mg2+ doping. Moreover, the photoluminescence of CZYP:0.12Dy3+ could be enhanced to a certain extent by adding quantitative Mg2+ and B3+. Through these tests, we observed that the phosphor of CMZYP:0.12Dy3+ was indeed a very suitable outdoor lighting material for environmental protection. Declaration of competing interest There are no conflicts to declare. Acknowledgment The authors would like to gratefully acknowledge funds from the National Natural Science Foundation of China (Grant No. 21706060, 51703061, 51974123), Hunan Provincial Engineering Technology Research Center for Optical Agriculture (Grant No. 2018TP2003) and Hunan Provincial Innovation Foundation for Postgraduate (Grant No. CX2018B396); Education Department of Hunan Province (Grant No. 19C0903); Science Fund Project of Hunan Agricultural University (Grant No. 19QN11). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ceramint.2020.01.203. References [1] J. Chen, N. Zhang, C. Guo, F. Pan, X. Zhou, H. Suo, X. Zhao, E.M. Goldys, Sitedependent luminescence and thermal stability of Eu2+ doped fluorophosphate toward white LEDs for plant growth, ACS Appl. Mater. Interfaces 8 (2016) 20856–20864. [2] Z. Zhou, Y. Li, M. Xia, Y. Zhong, N. Zhou, H.T.B. Hintzen, Improved luminescence and energy-transfer properties of Ca14Al10Zn6O35:Ti4+,Mn4+ deep-red-emitting phosphors with high brightness for light-emitting diode (LED) plant-growth lighting, Dalton Trans. 47 (2018) 13713–13721. [3] M.H. Fang, W.L. Wu, Y. Jin, T. Lesniewski, S. Mahlik, M. Grinberg, M.G. Brik, A.M. Srivastava, C.Y. Chiang, W. Zhou, D. Jeong, S.H. Kim, G. Leniec, S.M. Kaczmarek, H.S. Sheu, R.S. Liu, Control of luminescence by tuning of crystal symmetry and local structure in Mn4+ -activated narrow band fluoride phosphors, Angew. Chem. Int. Ed. 57 (2018) 1797–1801. [4] Y. Zhong, S. Gai, M. Xia, S. Gu, Y. Zhang, X. Wu, J. Wang, N. Zhou, Z. Zhou, Enhancing quantum efficiency and tuning photoluminescence properties in far-redemitting phosphor Ca14Ga10Zn6O35:Mn4+ based on chemical unit engineering, Chem. Eng. J. 374 (2019) 381–391. [5] X. Wu, L. Liu, M. Xia, S. Huang, Y. Zhou, W. Hu, Z. Zhou, N. Zhou, Enhance the luminescence properties of Ca14Al10Zn6O35:Ti4+ phosphor via cation vacancies engineering of Ca2+ and Zn2+, Ceram. Int. 45 (2019) 9977–9985. [6] J. Han, L. Li, M. Peng, B. Huang, F. Pan, F. Kang, L. Li, J. Wang, B. Lei, Toward Bi3+ red luminescence with No visible reabsorption through manageable energy interaction and crystal defect modulation in single Bi3+-doped ZnWO4 crystal, Chem. Mater. 29 (2017) 8412–8424. [7] J. Han, F. Pan, M.S. Molokeev, J. Dai, M. Peng, W. Zhou, J. Wang, Redefinition of crystal structure and Bi3+ yellow luminescence with strong near-ultraviolet excitation in La3BWO9:Bi3+ phosphor for white light-emitting diodes, ACS Appl. Mater. Interfaces 10 (2018) 13660–13668.
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[35] M. Chen, Z. Xia, M.S. Molokeev, T. Wang, Q. Liu, Tuning of photoluminescence and local structures of substituted cations in xSr2Ca(PO4)2–(1–x)Ca10Li(PO4)7:Eu2+ phosphors, Chem. Mater. 29 (2017) 1430–1438. [36] R. Chatterjee, S. Saha, D. Sen, K. Panigrahi, U.K. Ghorai, G.C. Das, K.K. Chattopadhyay, Neutralizing the charge imbalance problem in Eu3+-activated BaAl2O4 nanophosphors: theoretical insights and experimental validation considering K+ codoping, ACS Omega 3 (2018) 788–800. [37] T.W. Kuo, W.R. Liu, T.M. Chen, High color rendering white light-emitting-diode illuminator using the red-emitting Eu2+-activated CaZnOS phosphors excited by blue LED, Optic Express 18 (2010) 8187–8192. [38] H. Jing, C. Guo, G. Zhang, X. Su, Z. Yang, J.H. Jeong, Photoluminescent properties of Ce3+ in compounds Ba2Ln(BO3)2Cl (Ln = Gd and Y), J. Mater. Chem. 22 (2012) 13612–13618. [39] X. Min, M. Fang, Z. Huang, Y. Liu, Y. Huang, R. Wen, T. Qian, X. Wu, Enhanced thermal properties of novel shape-stabilized PEG composite phase change materials
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with radial mesoporous silica sphere for thermal energy storage, Sci. Rep. 5 (2015) 12964–12974. S. Liao, X. Ji, Y. Liu, J. Zhang, Highly efficient and thermally stable blue-green (Ba0.8Eu0.2O)(Al2O3)4.575x(1+x) phosphor through structural modification, ACS Appl. Mater. Interfaces 10 (2018) 39064–39073. G. Blasse, B.C. Grabmaier, Radiative return to the ground state: emission, Lumin. Mater. 96 (1994) 38–70. J. Xiang, J. Zheng, Z. Zhou, H. Suo, X. Zhao, X. Zhou, N. Zhang, M.S. Molokeev, C. Guo, Enhancement of red emission and site analysis in Eu2+ doped new-type structure Ba3CaK(PO4)3 for plant growth white LEDs, Chem. Eng. J. 356 (2019) 236–244. J. Qiao, L. Ning, M.S. Molokeev, Y.C. Chuang, Q. Liu, Z. Xia, Eu2+ site preferences in the mixed cation K2BaCa(PO4)2 and thermally stable luminescence, J. Am. Chem. Soc. 140 (2018) 9730–9736.
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Plant habitat-conscious phosphors: Tuneable luminescence properties of Dy3+-doped Ca8ZnY(PO4)7 phosphors by co-dopants Mg2+ and B3+ Yongli Zhanga,b,1, Minghui Lia,1, Zihui Konga, Chao Liangd, Cheng Zhoua, Mao Xiaa,b,c,∗∗, Zhi Zhoua,b,∗ a
College of Chemistry and Materials Science, Hunan Agricultural University, Changsha, 410128, PR China Hunan Optical Agriculture Engineering Technology Research Center, Hunan Province, 410128, PR China c State Key Laboratory of Powder Metallurgy, Central South University, Changsha, Hunan, 410083, PR China d Jiangsu Bree Optronics Co., Ltd., Nanjing, 211103, PR China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Devices Habitat-conscious Phosphor Dy3+-activator
Outdoor lighting and other lighting systems can disrupt natural plant growth habits. Thus, LED lighting that is not detrimental to plant growth is required. In our study, Dy3+-doped Ca8ZnY(PO4)7:Dy3+ phosphor with enhanced luminescence properties caused by the co-dopants Mg2+ and B3+ were synthesised. The samples had multiple excitation peaks, indicating they are excited by either near-ultraviolet (n-UV) or blue chips. All samples exhibited bright narrow yellow and blue emission corresponding to the transitions of Dy3+ ions with 4F9/ 6 4 6 2+ and B3+ enhanced the luminescence 2→ H13/2 and F9/2→ H13/2, respectively. Moreover, doping with Mg intensity, reaching 113.6 and 119.7%, respectively. In addition, the luminescence emission intensity at 150 °C was maintained at approximately 95% of the initial value at 25 °C, and its thermal stability increased by 123%. Devices assembled with an n-UV chip (388 nm) and the as-obtained CZMYP:Dy3+ phosphor emitted a bright warm white light and simulated outdoor dark lighting for tobacco cultivation, indicating that the as-prepared phosphor is an excellent candidate material for plant habitat-conscious phosphors.
1. Introduction Light is the source of energy required for plant growth and an important environmental signal regulating photosynthesis, material metabolism and gene expression [1–3]. The study of supplementary plant light is of great significance to economic development [4]. Compared with the ordinary fluorescent lamp, LED lamp have evident advantages: no mercury, simple structure, good seismic performance, high light efficiency, pure light quality, low energy consumption and other photoelectric advantages [5,6]. LED is recognised as the most promising electric light source in the 21st century, and it is widely used in the research of cultivated crops. The combination of the blue InGaN LED, yellow-emitting phosphor (Y3Al5O12:Ce3+), and red-emitting nitride phosphor is the most frequently used method to package LEDs [7]. However, the preparation conditions for these nitride phosphors are harsh, as they often require a high temperature (> 1800 °C) and high pressure in an oxygen-free environment [8–10]. Moreover, The combination of the blue InGaN LED chip and the two types of phosphor can
lead to waste of materials or uneven mixing, causing a reduction in the quantum efficiencies of samples [4]. A large Stokes shift and the intense reabsorption of blue light by red phosphors are the possible reasons. However, these problems are easily solved by the single-phase phosphor because of higher luminous efficiency, small colour aberration, excellent colour rendering index (CRI), and low cost. Most researchers have focused on white LEDs (WLEDs) that associate with people's colour perception. However, the fluorescent lamps that emit light brighter light have negative effects on the natural environment [11]. At night, the prolonged exposure of ordinary WLED lights can inhibit the flowering and fruiting of short-day plants and promote the flowering and fruiting of long-day plants, especially in caves and forests and roadsides where many lights are used [12]. Finally, the growth of plants is affected, natural plant habitats are destroyed, and crop yields decrease. Phosphor converted LEDs, as the primary artificial light source, are fabricated by combining various chips and matched phosphors [13]. The luminescence properties of assynthesised phosphor materials are significant parameters LED lighting
∗
Corresponding author. College of Chemistry and Materials Science, Hunan Agricultural University, Changsha, 410128, PR China. Corresponding author. College of Chemistry and Materials Science, Hunan Agricultural University, Changsha, 410128, PR China. E-mail addresses: [email protected] (M. Xia), [email protected] (Z. Zhou). 1 Yongli Zhang and Minghui Li contributed equally to this work. ∗∗
https://doi.org/10.1016/j.ceramint.2020.01.203 Received 24 December 2019; Received in revised form 19 January 2020; Accepted 21 January 2020 0272-8842/ © 2020 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Yongli Zhang, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2020.01.203
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thermal stability of the phosphor were studied detail. Furthermore, the corresponding reasons and enhancement mechanism are investigated.
applications for plant growth. Thus, changing the impact of outdoor lighting on local plant local species by changing the properties of phosphors is necessary. Dysprosium is considered a familiar activator with abundant energy levels, high colour purity, and high conversion and luminescence efficiency for the design of suitable phosphors [14]. For example, Nakajima studied the effects of WLEDs made of phosphors doped with Dy3+ on the photosynthesis of chlorella and observed that these lamps inhibited the intensity of photosynthesis to 26% due to the reduction of red-nearIR emissions [11]. This also proves that compared with ordinary LED lights, these WLEDs can indeed reduce the photosynthesis of plants at night. The luminescent materials of phosphate hosts have the advantages of structural diversity, excellent thermal stability and steadiness of ionic charge in the lattice [15]. Many reports indicate that (β-Ca3(PO4)2)-type as a potential whitlockite-type crystal texture compounds have five types of Ca sites, namely, Ca1–Ca5 [16,17]. All except Ca4 are completely filled, and the Ca4 site is only half occupied. We can assume a crystal structure whose position is completely filled or completely empty, because its stable structure is related to the half-occupied Ca4 [18–20]. This particular crystal texture allows abundant heterovalent substitution by forming vacancies. In general, Ca2+ can be replaced with a univalent ion M+ (Li or Na), a bivalent ion M2+ (Mg or Zn), and a trivalent ion R3+(R = Y, Ga, In) in the structure of β-Ca3(PO4)2 to obtain different compounds [18]. Teterskii et al. researched the properties of Ca9-xMxR (PO4)7(M = Mg, Zn; R = Ln, Y; 0 < x < 1.5) which is also a new family of whitlockite-type hosts [21]. Additionally, Jang et al. studied the luminescence of Ca8MgR(PO4)7:Eu3+ (R = La, Gd, Y) on the basis of Teterskii's study [19]. We have recently demonstrated the great thermal stability of a novel Ca8ZnY(PO4)7:Dy3+ phosphor. However, although it was successfully synthesised, the luminescent property was still not sufficiently high to meet the demand for high-power lighting applications. Therefore, improving the phosphors property by various methods is still necessary. The luminescence properties of as-synthesised phosphor materials are significant parameters for plant-growth LED lighting applications [22,23]. Enhancing luminescence properties is mainly achieved through the following three methods: 1) charge compensation. This can modify the composition of defects to construct an optimal activator environment or alter the local site symmetry of the host structure by introducing a foreign ion [24,25]. For instance, Peng et al. reported that charge compensator ions (Li+, Na+, and K+) in SrAl2O4:Eu3+ phosphor enhance its luminescence properties [26]; 2) adding a fluxing agent. This can decrease the probability of non-radiation transition and sintering temperature for energy conservation [27,28]. The emission intensity and quantum efficiency of Lu3-xAl5O12:xCe3+ phosphor can be enhanced by adding various fluxing agent, and BaF2 (3 wt%) exhibited the best effect [29]; 3) the solid solution effect. This plays a key role in the design and preparation of Dy3+-doped Ca8(Zn/Mg)Y(PO4)7 phosphors. The solid solution effect mainly replaces the lattice position of the host ion by introducing a foreign ion, causing enhanced thermal stability and quantum efficiencies of samples via changing the crystal local structure of the matrix [30–32]. For instance, Qiao et al. reported a red emission (Ba1−xSrx)2YSbO6:Mn4+ phosphor through tuning of the compositions and multiple activator sites, and the thermal stability increased significantly via Sr2+ replacing Ba2+ [33]. Analogously, the thermal stability of Ca3−xSrx(PO4)2:Eu2+ phosphor is improved based on the solid solutions method [34]. Therefore, we assumed Mg2+ replaced Zn2+ in Ca8ZnY(PO4)7:Dy3+ phosphor to enhance its thermal stability and luminescence properties. In this work, Ca8ZnY(PO4)7:Dy3+, Ca8Zn1–yMgyY(PO4)7:Dy3+ and boron-ion-co-doped Ca8Zn1–yMgyY(PO4)7:Dy3+, z%wtB3+ phosphor were prepared through the high-temperature approach. First, the luminescence behaviour and mechanism of Dy3+-activated Ca8ZnY(PO4)7 phosphor are discussed. In addition, the effect of doping Mg2+ ion and co-doping with B3+ ion on the photoluminescence (PL) properties and
2. Experimental sections 2.1. Materials Ca8ZnY1-x(PO4)7:xDy3+ (CZYP:Dy3+; x = 0.06, 0.09, 0.12, 0.15, and 0.18, molar ratio) and Ca8(Zn1–yMgy)Y(PO4)7:Dy3+ (CZMYP:Dy3+; y = 0, 0.2, 0.4, 0.6, 0.8, and 1.0 M ratio) phosphors were compounded using the traditional high temperature method. The original reagents were CaCO3 (AR), Dy2O3 (99.99%), NH4H2PO4 (99.99%), MgO (99.99%), Y2O3 (99.99%) and ZnO (99.99%) bought from Aladdin Reagent (Shanghai) Co., Ltd, and different contents of H3BO3 (AR) acted as the flux in the reaction process to obtain the ideal samples. All initial materials were weighted proportionately and the mixtures were thoroughly ground in corundum crucibles for 30 min. Subsequently, at the heating rate of 5 °C/min, powders were heated up to 1300 °C for 5 h in the ambient atmosphere. After sintering, the furnace was closed to let the samples to cool to the ambient temperature. Finally, the samples were removed and carefully pulverised into fine powders for further analysis and characterisation. 2.2. Measurement The X-ray powder diffraction (XRD) patterns ranging from 10 to 80° were obtain using a diffractometer (D/Max-2200/pc, Rigaku); the sealed Cu-Kα X-ray tube was set at 40 kV and 20 mA. The elemental mapping images and compositional elements analysis of the CZYP:0.12Dy3+ phosphor were recorded using an energy dispersive Xray detector (EDX). The UV–vis absorption spectra were recorded on the U-3310 spectrophotometer (Hitachi, Japan). An F-4700 fluorescence spectrophotometer (Hitachi, Japan) equipped with an excitation light source (150W Xe lamp) measured the photoluminescence excitation (PLE), PL spectra and PL spectra with temperature variation. The decay times for the prepared phosphors were measured on an FLS920 spectrometer (Edinburgh, UK). The phosphors were coated on blue chips with a 470 nm emission. The electro-luminescence (EL) spectra and parameters of the pc-LED devices were measured by a LED spectrophotocolorimeter (ATA-500, Everfine). 3. Results and discussion 3.1. Crystal structure and purity analysis The XRD patterns of the Dy3+-doped CZYP phosphors are illustrated in Fig. 1b. The results of one study indicate that Ca9-xMxR(PO4)7 (x = 1–1.5; M = Mg, Zn, and Cd; R = Ln and Y) is iso-structural to Ca9Ln(PO4) [21]. The phase purity of the phosphor was analysed by comparing them with the standard card of PDF#49–0503; the as-prepared phosphors indicated they were almost crystallised in a purity phase with no structural changes or impurity peaks. When a small quantity of Dy3+ replaced Y3+ in the CZYP matrix, the XRD patterns maintained the original position without any shift. However, the XRD patterns indicated a move to a higher angle when Mg2+ replaced Zn2+; the experimental results are shown in Fig. S1. Typically, the CZYP phosphor has a the hexagonal structure with an R3c(161) space group [35] (in Fig. 1a). The structure parameters of the compound were a = 10.40080 Å, b = 10.40080 Å, c = 37.27200 Å, V = 3491.7793 Å3, and Z = 6. Ca was observed to be surrounded by O atoms and [PO4] tetrahedrons (in Fig. 1c). Three coordination environments surrounded the O atoms by 8, 8, and 9, respectively. Fig. 2 depicts the elemental mapping images of the CZYP:Dy3+ sample. The figure distinctly shows that the components of Ca, O, Y, Dy, P, and Zn were distributed in the target particle evenly, indicating that Dy successfully into the crystalline structure of the CZYP host. In 2
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Fig. 1. (a) XRD pattern of as-prepared CZYP:Dy3+ phosphors; (b) The crystal structure of CZYP compound.
concentration quenching effect. Blasse pointed out that the non-radiative energy transfer in the process of luminescence of oxalic phosphors is due to the resonance transfer of electric multipoles or exchange interaction. Additionally, the mechanism is determined by calculating the critical distance (RC) between the neighbouring Dy3+ ions. There are two scenarios: when the distance of Dy–Dy is less than 5 Å, the exchange interaction is effective, and when the Dy–Dy distance is greater than 5 Å, the most interaction is multipolar. According to Blasse, RC can be calculated through the following formula [2,5,36,37]:
addition, the calculated atomic ratio was approximate to the desired formula, indicating the successful synthesis of the target phosphor. 3.2. Luminescence properties of CZYP:xDy3+ Fig. 3a depicts the UV–vis absorption spectrum of CZYP, CZYP:0.12Dy3+ and CMZYP:0.12Dy3+ samples. The figure shows that all specimens exhibited narrow absorption bands in the 200–250 nm wavelength range, which were caused by the host absorption of CZYP. In the wavelength range of 250–350nm, the absorption intensity of CZYP and CMZYP:0.12Dy3+ tended to decline compared with the previous band. This can be clearly understood because the Dy3+-doping enhanced the absorption intensity. In addition, the peak positions of excitation spectrum match with the absorption spectrum. In general, CZYP:0.12Dy3+ phosphors have a strong absorption capacity, indicating that the addition of Dy3+ is beneficial to the photoluminescence of CZYP:0.12Dy3+. To better understand the band structure of the CZYP:0.12Dy3+ sample, we calculated the band gap energies of CZYP, CZYP:0.12Dy3+ and CMZYP:0.12Dy3+ samples using the following formula [4,31]:
(αhν )2 = A (hν − Eg )
1
3V ⎤ Rc ≈ 2 ⎡ ⎢ 4πχ c N ⎥ ⎣ ⎦
3
(2) 3+
In this formula, Xc represents the critical concentration of Dy , V represents unit cell volume, and N is the number of dopant effective sites per cell. In this study, N was 6, V is 3491.7793 Å3, and Xc was 0.12. Finally, the RC of the CZYP:Dy3+ is determined as approximately 21 Å. Moreover, the value of θ represents dipole-dipole interactions (θ = 6), dipole-quadruple interactions (θ = 8) and quadruple-quadruple interactions (θ = 10), respectively. In general, the three types can be determined by the value of Rc, such as the exchange interaction (Rc < 5 Å). On the basis of Van Uiter's theory, three forms of energy transfer occur in multipolar interactions. The form of multipolar interaction can be decided according to the intensity variation of the emission level with multipolar interactions. The relationship between an activator ion (x) and emission intensity (I) is as follows [1,38,39]:
(1)
where h stand for the Planck constant, α is the absorption coefficient, v is the frequency constant, and Eg represents the band gap energy. As shown in Fig. 3b, the specific values of the band gap energies of CZYP, CZYP:0.12Dy3+, and CMZYP:0.12Dy3+ were determined to be 4.63, 4.28, and 4.54 eV, respectively. As the figure shows, the energy gap of the CZYP host with Dy3+ doping decreased from 4.63 to 4.28 eV, which was beneficial to the luminescence properties of CZYP:Dy3+ and exhibited excellent thermal stability through reducing the possibility of photoionisation. The excitation and emission spectra of CZYP:xDy3+ (x = 0.06, 0.09, 0.12, 0.15, and 0.18) are investigated as displayed in Fig. 4. The effect of diverse doping concentrations of Dy3+ on PL intensity is shown in Fig. 4a. As the figure shows, when the critical concentration of Dy3+ was 0.12, the emission intensity reached its maximum value. The PL intensity decreased with increase in Dy3+ content because of the
I / x = K [1 + β (x )Q/3]−1
(3)
Here, I represent the PL intensity, x represents the concentration of Dy3+ ions, β and K represents constants of the CZYP structure, respectively. Q is a specific constant whose different values reflect the energy transfer patterns of multipolar interaction (6 for d-d, 8 for d-q, and 10 for q-q). Fig. 4c shows the linear relationship between log (I/x) and log (x), and the slope of the fitting line was –1.069. Finally, Q was calculated as approximately 3.207, indicating that the main mechanism of Dy3+ ions concentration quenching was a dipole-dipole interaction. At the same time, excitation spectra of CZYP:0.12Dy3+ were 3
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Fig. 2. (a)–(g) The elemental mapping images of total elements, O, Ca, Y, P, Dy and Zn, respectively; (h) The corresponding energy-dispersive X-ray spectroscopy (EDS) measurement of CZYP:0.12 Dy3+ phosphor.
monitored as shown in Fig. 4b. The figure shows some main peaks located at 324,340, 350, 362, 388, 425,450, and 473 nm and were assigned to the transitions from 6H15/2→4M17/2, 4I9/2, 6P7/2, 6P5/2, 4I13/2, 4 G11/2, 4I15/2 and 6F9/2, respectively, because of the specific electronic configuration of Dy3+. To better understand the influence of Dy3+ concentration on luminescence performance, we selected samples with doping concentrations of 0.6 and 0.12 and their decay curves were tested under the conditions of 288-nm excitation and 574 nm monitoring. Fig. 4d shows the Ca8ZnY1-x(PO4)7:xDy3+ (x = 0.6, x = 0.12) phosphors PL decay curves. Moreover, the decay data of specimens can be fitted well using the model of single exponential decay, which is shown below [13,40,41]:
−t I = I0 × exp ⎛ ⎞ ⎝ τ ⎠
killer locus increased. As a result, the lifetimes decreased with increasing Dy3+ concentrations. The luminescence schematic diagram of the Dy3+-doped CZYP substrate is shown in Fig. 5. First, many electrons were excited under ultraviolet light and some holes were formed. Because the large radiant energy was greater than the band gap energy, an electron could jump immediately from the valence band to the conduction band. The high energy level original electron group relaxed to 4F9/2 by the process of non-radiative relaxation (NR). Eventually, all the electrons could be radiated back to the ground state. Two distinct centres of light corresponded to the two types of transitions of the magnetic dipole (4F9/2 →6H15/2) and electric dipole (4F9/2→6H13/2). The magnetic dipole of the 4F9/2 to 6H15/2 transition (blue emission) is known to dominate when Dy3+ ions are in highly symmetric positions with a centre of inversion. Moreover, when Dy3+ ions are in low symmetric positions with no inversion centres, the electric dipole (4F9/2→6H13/2) transition (yellow emission) is dominant. In the experiment, the PL intensity at 574 nm (yellow emission) was stronger than the 484 nm blue emission, indicating that Dy3+ ions in this model occupied a low-symmetry site.
(4)
where τ and t represent luminescence lifetimes and time, and I and I0 stand for the luminescence intensities at t and 0. When the doping concentrations of Dy3+ were 0.06 and 0.12, the lifetimes were 0.512 and 0.425 ms, respectively. As the concentration of the Dy3+ increased, the distance of Dy―Dy decreased. Therefore, the rate of energy transfer between Dy―Dy and the energy transfer possibility to the luminous 4
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3.3. Enhanced luminescence of CZYP:0.12 Dy3+ phosphor through doping Mg2+ and B3+ Here, tuning of the emission intensity through co-doped Mg2+ and B3+ ions is discussed to further improve the luminescence intensity of the samples, as shown in Fig. 6. This can be seen in the illustration in Fig. 6a. The PL intensity of is maximum when the Mg2+ ions doping content is 0.6 and the photoluminescence intensity of phosphor increased by 113.6%. Moreover, adding a H3BO3 fluxing agent could enhance the photoluminescence intensity of the phosphors. In Fig. 6b, when the B3+ ions doping amount was 1.0%, the emission intensity was the highest. The photoluminescence intensity was detected with a 119.7% improvement in this case. The results indicated that the luminescence performance of CZYP:0.12Dy3+ phosphors could be significantly improved by co-doping Mg2+ and B3+ into the crystal structure. The two substances can enhance the luminescence intensity because they can alter the crystal environment of activator ions, promote the activator ions to enter the matrix lattice, control the size of phosphor particles and enhance the luminescence intensity.
3.4. Thermal stability of CZYP:0.12 Dy3+ and CZMYP:0.12 Dy3+ phosphors Thermal stability is known to be a significant index in checking if phosphors can be used in practice because LEDs operate at a higher working temperature than 25 °C. Fig. 7a shows the 3D temperaturedependent PL spectra of the CZYP:0.12Dy3+ phosphors, with the main emission peaks located at approximately 484 and 574nm, due to energy level transition (4F9/2→6H15/2, and 6H13/2). We can clearly see that like for most phosphors, the luminescence intensity of CZYP decreased with increasing temperature, which was related to the thermal quenching. However, based on the previous research of enhancing luminescence, we observed an interesting phenomenon. When the Mg2+ ion doping content was 0.6, the thermal stability of phosphor at 150 °C was 95%, an 8% increase from that of the un-doped phosphor. High thermal stability also provides the premise for the application of high-power LED. Moreover, activation energy (Ea) is widely used to test thermal
Fig. 3. (a) The UV–vis absorption spectra of CZYP and CZYP:Dy3+. (b) The band gap calculated based on above absorption data.
Fig. 4. (a) The concentration-dependent emission spectra of CZYP:xDy3+ (x = 0.06, 0.09, 0.12, 0.15, 0.18); (b) the excitation spectra of CZYP:0.12 Dy3+; (c) The plot of Log(I/x) as a function of Log(x); (d)decay curves of CZYP:xDy3+ phosphors. 5
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Fig. 5. Emission spectra of different contents of (a–d) H3BO3, Li+, Na+ and K+ (n = 0–1.5, x = 0–1.5, y = 0–1.5, z = 0–1.5) co-doping CSYAO:1.2%Mn4+ phosphors.
Fig. 6. Energy level and electron transitions schematic diagram of Dy3+-doped CZYP host.
Fig. 7. (a) 3D temperature-dependent PL spectra of CZYP:0.12Dy3+ phosphors; (b) The emission spectra of CZYP:0.12Dy3+ and vurse temperature CMZYP:0.12Dy3+ (25–200 °C); (c) The relationship between emission intensity and temperature of phosphors; (d) Arrhenius fitting of emission and intensity of CZYP: 0.12Dy3+ 3+ CMZYP:0.12 Dy .
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Fig. 8. The emission spectra of CZYP:0.12Dy3+ (a) and CMZYP:0.12Dy3+ (b) vurse temperature (25–200 °C). (c) The coordinate diagram of excited and ground states of Dy3+.
resistance of phosphors. For the same phosphors, a higher Ea value signifies stronger thermal stability. The following formula is used to compute Ea [42,43]:
ln[(I0/ I ) − 1] = −Ea/kT + c
(5)
Here, I represent the emission intensity at T and I0 is the PL intensity at indoor temperature. C and T denote a constant and Kelvin temperature, respectively. And k represents Boltzmann constant (k = 8.62 × 10−5 eV) Fig. 7c shows the linear relation between I/kT and In (I0/I–1). The slope of CZYP:0.12 Dy3+ was – 0.322, and Ea was determined to be 0.322eV. The slope of CMZYP:0.12 Dy3+ was – 0.381, and Ea was determined to be 0.381eV, which was larger than that of CZYP. Therefor adding 0.6 Mg improves the thermal stability of the samples. In addition, we created a comparison chart. Fig. 8a and b show the comparison of emission spectra of two specimens with the change in temperature. The figure clearly shows that, compared with CZYP host, the decreasing tendency in CMZYP with temperature change is particularly slow, and the emission intensity of Dy3+ is still very high even at 200 °C. In Fig. 8c, the different curves for 4I13/2—4I15/2—4F9/2 and 6 H9/2—6H11/2—6H13/2—6H15/2 represent the excited and ground states of Dy3+, respectively. A is the bottommost dot at 4F9/2. The curves representing the excited states and ground states intersect at point M. Under UV excitation at 388 or 452nm, electrons in the energy level (6H9/2, 6H11/2, 6H13/2 and 6H15/2) are first excited into the excited states at 25 °C. Additionally, through radiative transition, many electrons in 4 I13/2, 4I15/2, or 4F9/2 moved from point A to the ground states. As the temperature increased, the electronically excited state may have conquered the activation energy(ΔE1)because of strong phonon-electron coupling and straight into tunnel D. Therefore, ΔE1 is related to the thermal quenching. 3.5. Optical characteristics and LED manufacturing To confirm the potential application of phosphors, we tested the chromaticity coordinates and LED lamp of CZYP:0.12Dy3+, as shown in Fig. 9a. A standard single phase white luminescent phosphor that is suitable for outdoor lighting is realised. The electroluminescence spectra of the white LED lamps based on a 388nm blue-chip and CZYP:Dy3+ phosphor and driven with a current of 150 mA are shown in Fig. 9b. The LED device including CZYP:Dy3+ phosphor exhibited a stronger yellow and blue emission in the range of 484, 574nm. As expected, the emission region does not match with the plant pigments
Fig. 9. (a) CIE chromaticity coordinate of CZYP:Dy3+. (b)The electro-luminescent (EL) spectra of as-fabricated pc-LED device with the CZYP:Dy3+ phosphor and the absorption curves of Chlorophyll B and phytochrome PFR. The inset shows the appearance of as-packaged LED apparatus and the photograph in operation at current of 150 mA.
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absorption, and the phosphor can be used to effectively inhibit plant photosynthesis and protect natural plant habitats.
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4. Conclusion A high stability single-phase white emission CZYP:Dy3+ phosphor was prepared generally using the high-temperature method. The phosphors had a rich electron transitions because Dy3+ has a special 4f orbital. Excited at 388nm, two main emission spectra, 484 and 574nm, were detected corresponding to the two types of transition (4F9/ 6 6 2→ H15/2, and H13/2), respectively. Because dipole-dipole interaction 3+ of Dy caused concentration quenching, thus the best doping of Dy3+ was 0.12 and the CIE chromaticity coordinates were (0.3326, 0.3862). According to the test, the luminous intensity of the sample could retain 87% of the room temperature when warming up to 150 °C, and the thermal stability of phosphor increased to 95% with Mg2+ doping. Moreover, the photoluminescence of CZYP:0.12Dy3+ could be enhanced to a certain extent by adding quantitative Mg2+ and B3+. Through these tests, we observed that the phosphor of CMZYP:0.12Dy3+ was indeed a very suitable outdoor lighting material for environmental protection. Declaration of competing interest There are no conflicts to declare. Acknowledgment The authors would like to gratefully acknowledge funds from the National Natural Science Foundation of China (Grant No. 21706060, 51703061, 51974123), Hunan Provincial Engineering Technology Research Center for Optical Agriculture (Grant No. 2018TP2003) and Hunan Provincial Innovation Foundation for Postgraduate (Grant No. CX2018B396); Education Department of Hunan Province (Grant No. 19C0903); Science Fund Project of Hunan Agricultural University (Grant No. 19QN11). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ceramint.2020.01.203. References [1] J. Chen, N. Zhang, C. Guo, F. Pan, X. Zhou, H. Suo, X. Zhao, E.M. Goldys, Sitedependent luminescence and thermal stability of Eu2+ doped fluorophosphate toward white LEDs for plant growth, ACS Appl. Mater. Interfaces 8 (2016) 20856–20864. [2] Z. Zhou, Y. Li, M. Xia, Y. Zhong, N. Zhou, H.T.B. Hintzen, Improved luminescence and energy-transfer properties of Ca14Al10Zn6O35:Ti4+,Mn4+ deep-red-emitting phosphors with high brightness for light-emitting diode (LED) plant-growth lighting, Dalton Trans. 47 (2018) 13713–13721. [3] M.H. Fang, W.L. Wu, Y. Jin, T. Lesniewski, S. Mahlik, M. Grinberg, M.G. Brik, A.M. Srivastava, C.Y. Chiang, W. Zhou, D. Jeong, S.H. Kim, G. Leniec, S.M. Kaczmarek, H.S. Sheu, R.S. Liu, Control of luminescence by tuning of crystal symmetry and local structure in Mn4+ -activated narrow band fluoride phosphors, Angew. Chem. Int. Ed. 57 (2018) 1797–1801. [4] Y. Zhong, S. Gai, M. Xia, S. Gu, Y. Zhang, X. Wu, J. Wang, N. Zhou, Z. Zhou, Enhancing quantum efficiency and tuning photoluminescence properties in far-redemitting phosphor Ca14Ga10Zn6O35:Mn4+ based on chemical unit engineering, Chem. Eng. J. 374 (2019) 381–391. [5] X. Wu, L. Liu, M. Xia, S. Huang, Y. Zhou, W. Hu, Z. Zhou, N. Zhou, Enhance the luminescence properties of Ca14Al10Zn6O35:Ti4+ phosphor via cation vacancies engineering of Ca2+ and Zn2+, Ceram. Int. 45 (2019) 9977–9985. [6] J. Han, L. Li, M. Peng, B. Huang, F. Pan, F. Kang, L. Li, J. Wang, B. Lei, Toward Bi3+ red luminescence with No visible reabsorption through manageable energy interaction and crystal defect modulation in single Bi3+-doped ZnWO4 crystal, Chem. Mater. 29 (2017) 8412–8424. [7] J. Han, F. Pan, M.S. Molokeev, J. Dai, M. Peng, W. Zhou, J. Wang, Redefinition of crystal structure and Bi3+ yellow luminescence with strong near-ultraviolet excitation in La3BWO9:Bi3+ phosphor for white light-emitting diodes, ACS Appl. Mater. Interfaces 10 (2018) 13660–13668.
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