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Crystal structure and luminescence properties of CaTiO3:Dy3+ phosphor co-doped with Zr4+
Optical Materials 98 (2019) 109446
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Optical Materials journal homepage: http://www.elsevier.com/locate/optmat
Crystal structure and luminescence properties of CaTiO3:Dy3þ phosphor co-doped with Zr4þ Qian Yin a, Kehui Qiu b, *, Wentao Zhang a, Xianfei Chen a, Peicong Zhang a, Qinxue Tang a, Min Chen a a b
College of Materials and Chemistry & Chemical Engineering, Chengdu University of Technology, Chengdu, 610059, China Institute of Materials Science and Technology, Chengdu University of Technology, Chengdu, 610059, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Combustion Ca(Ti1 xZrx)O3:Dy3þ WLEDs Structural substitution
A series of Ca(Ti1 xZrx)O3:Dy3þ phosphors were fabricated using a combustion method. The lattice structure, morphological features, and luminescence performances of the specimens were obtained. Rietveld refinement of the XRD patterns confirmed the crystal distortion of CaTiO3 after Zr4þcodoping and the positional occupation of Zr4þ in the CaTiO3 crystal lattice. The photoluminescence spectroscopy results indicated that the fluorescence emission intensity of CaTiO3:Dy3þ was substantially enhanced by Zr4þ codoping and that the optimum doping concentration of Zr4þ was 40% mol. The quantum yield of the Ca(Ti0⋅6Zr0.4)O3:Dy3þ phosphor was measured to be 9.9%, and its fluorescence lifetime decreased with the increasing concentration of Zr4þ doping. Furthermore, the lowest color temperature was observed for Ca(Ti0⋅6Zr0.4)O3:Dy3þ among the investigated phosphors and its color coordinate was the closest to the standard white-light color coordinate. These results suggest that the Ca (Ti0⋅6Zr0.4)O3:Dy3þ phosphor excited by near-ultraviolet light is a potential material for application in white light-emitting diodes.
1. Introduction White light-emitting diodes (WLEDs) are currently regarded as the most extensively used solid-state lighting to replace incandescent and fluorescent lamps. They exhibit numerous advantages compared with incandescent and fluorescent lamps, including lesser environmental impact, longer lifetimes, enhanced safety, and higher efficiency [1–4]. Currently, the combination of a light-emitting diode (LED) chip and phosphor is considered to be the most effective approach for generating white light. For instance, in commercial devices, a blue GaN LED chip is combined with the Y3Al5O12:Ce3þ yellow phosphor [5,6]. However, this approach often causes defects. For example, over time, aging of the chip can decrease the color rendering index and instability of the color temperature because of a lack of red emission. In addition, blue LED chips can emit excessive blue light. Biological and medical researchers have reported that excessive blue light increases the risk of psycholog ical or physiological issues such as depression and sleep disorders [7–9]. Another approach to generate white light is to combine semiconductor chips that emit near-ultraviolet (NUV) light with tricolor phosphors. However, multiphase phosphors reduce the luminous efficiency in
practical applications because of the differences in decay times and the reabsorption of emission colors. A single-composition phosphor that emits white light under NUV excitation and binds to LEDs in a potential solid-state lighting material would overcome these deficiencies [10,11]. Calcium titanate (CaTiO3) is the representative perovskite structure exhibiting a general formula of ABO3, where A is a cation with a large ionic radius like Ca2þ and B is a cation with a smaller radius, mainly Ti4þ or Nb5þ [12]. CaTiO3 presents excellent chemical stability, electrical conductivity, and biocompatibility. Because of these advantages, CaTiO3 is extensively used in high-frequency dielectric MLC ceramic materials, dielectric resonators and luminescent materials [13–15]. Recently, CaTiO3 has been proposed as a luminescent phosphor host material because of its remarkable properties [16–19]. Moreover, the f–f transition of the Dy3þ causes it to produce two emission peaks, located at 578 nm (yellow) and 484 nm (blue) matching with the 4F9/2 → 6H13/2 and 4F9/2 → 6H15/2 transitions of the Dy3þ, respectively, which are excited by NUV [20]. The yellow-light and blue-light can be combined to come into being white light under NUV excitation [21]. Based on the aforementioned advantages, CaTiO3:Dy3þ phosphors have been recently proposed. However, their emission intensity is
* Corresponding author. E-mail address: [email protected] (K. Qiu). https://doi.org/10.1016/j.optmat.2019.109446 Received 4 September 2019; Received in revised form 8 October 2019; Accepted 8 October 2019 Available online 17 October 2019 0925-3467/© 2019 Elsevier B.V. All rights reserved.
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Fig. 1. X-ray diffraction patterns of Ca(Ti1 xZrx)O3:Dy3þ (0 � x � 60%) synthesized at 900 � C.
mixed in a beaker. The mixed solution in the beaker was subsequently stirred using a magnetic at the temperature of 80 � C. After the mixture became clear, anhydrous citric acid was added and the resulting mixture was stirred for approximately 20 min. The previously prepared Ca (NO3)2 and Dy(NO3)3 aqueous solutions were subsequently added into the aforementioned mixture; the resulting mixture was stirred until it formed a buff sol precursor. Finally, the obtained specimen was sintered at 900 � C for 1 h in a high temperature muffle furnace to gain a white powder.
Table 1 The lattice parameters of the PDF cards. Chemical formula
PDF card
Space group
a (Å)
b(Å)
c(Å)
v(Å3)
CaTiO3
82–0229
5.4086
5.4553
7.6782
226.55
CaZrO3
76–2401
Pbnm (62) Pcnm (62)
5.5912
8.0171
5.7616
258.26
inadequate to satisfy the application requirements. Therefore, re searchers have been attempting to increase the emission intensity of the CaTiO3:Dy3þ phosphors through charge compensation, crystal distor tion and structural substitution [22,23]. Because the Zr4þ has a coor dination number of six and can form an octahedron with six O2 , similar to the octahedral structure of the Ti4þ, we proposed that codoping Zr4þ into the CaTiO3:Dy3þ phosphor would enhance its luminescence properties. Herein, a series of Ca(Ti1 xZrx)O3:Dy3þ phosphors were synthesized using a combustion method for the first time. Further, the changes in the crystal parameters of these samples with variations in the concentration of Zr4þ were investigated and the position of Zr4þ in the CaTiO3 crystal structure was discussed. In addition, the effects of Zr4þ on the photo luminescence characteristics and crystal lattice structure of these Zr4þ doped phosphors were analyzed.
2.2. Sample characterization Phosphors were structurally characterized by X-ray diffraction (XRD) with Cu Kα radiation (λ ¼ 1.5418 Å, 40 mA, 40 kV Philips X’Pert Pro MPD). The morphology of the as-synthesized samples was observed via scanning electron microscopy (SEM) (model JEX-100CX, magnifi cation 20,000 � , voltage 10 kV). The compositional analysis of the samples was conducted using energy-dispersive X-ray spectroscopy (EDS). Further, the photoluminescence performances of the specimen were tested by a spectrophotometer (PL Hitachi F-4600) assembled with a Xe lamp (150-W) operated at 500 V. The quantum yield (QY) of the phosphor was obtained through FSL 980 (Edinburgh). All the samples were measured at ambient temperature. 3. Results and discussion
2. Experimental
3.1. Formation and morphology
2.1. Sample synthesis
Fig. 1 depicts the XRD of Ca(Ti1 xZrx)O3:Dy3þ (0 � x � 60%) phos phors acquired at 900 � C using the combustion method. The patterns of all the samples indicated an orthorhombic perovskite-like structure. However, the crystal parameters changed as the chemical composition of the samples was varied. The diffraction peaks of the obtained speci mens completely fitted with the standard diffraction site and intensity of CaTiO3 (PDF card 82–0229). Specifically, as x was increased from 0 to 60%, the synthesized matrix material gradually shifted from CaTiO3 to CaZrO3 (PDF card 76–2401). All these standard PDF cards refer to ABO3 perovskite-type oxides in which A denotes the Ca atoms and B denotes the Ti/Zr atoms. One B4þ and six O2 coordinate to form a BO6
3þ
A series of Ca(Ti1 xZrx)O3:Dy (x ¼ 0, 10%, 20%, 30%, 40%, 50%, 60%) phosphors were synthesized via the combustion method. The raw materials were Ca(NO3)2⋅4H2O (AR grade), Ti(OC4H9)4 (AR grade), (CH2OH)2 (AR grade), Zr(NO3)4⋅5H2O (AR grade), HNO3 (AR grade), absolute ethanol (AR grade), anhydrous citric acid (AR grade), and highpurity Dy2O3 powder (99.9%). First, Dy2O3 was dissolved in a mixture of 0.2 mL of HNO3 and 3 mL of distilled water. Then adding the Ca (NO3)2⋅4H2O in the mixed solution. Subsequently, 3.5 mL of Ti(OC4H9)4 and 6.2 mL of (CH2OH)2 were measured using a graduated cylinder and 2
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Fig. 2. The Rietveld profile fits the diffraction data of Ca(Ti1 xZrx)O3:Dy3þ (x ¼ 0, 20%, 40%, 60%). Table 2 The lattice parameter of the Ca(Ti1-xZrx)O3:Dy3þ(x ¼ 0, 20%, 40%, 60%) after refinement. Chemical formula 3þ
CaTiO3: Dy Ca(Ti0⋅8Zr0.2)O3: Dy3þ Ca(Ti0⋅6Zr0.4)O3: Dy3þ Ca(Ti0⋅4Zr0.6)O3: Dy3þ
a (Å)
b(Å)
c(Å)
v(Å3)
5.3902 5.4368 5.4982 5.5895
5.4437 5.5239 5.6394 7.7934
7.6052 7.7206 7.8075 5.1579
223.1593 231.8692 242.0832 224.6828
Table 3 Coordinates of each atom in Ca(Ti1-xZrx)O3: Dy3þ (x ¼ 40%) phosphor and its occupancy rate. Atoms
x
y
z
Occupancies
Uiso
Ca O1 O2 Ti Zr Dy
0.026718 0.079816 0.215062 0.000000 0.000000 0.026718
0.526757 0.031634 0.300832 0.000000 0.000000 0.526757
0.250000 0.250000 0.043354 0.000000 0.000000 0.250000
0.985581 1.000000 1.000000 0.560678 0.439322 0.014418
0.025109 0.039563 0.049757 0.020123 0.020123 0.025109
Fig. 3. The crystal structure of the standard CaTiO3 (a) and the Ca(Ti1 xZrx)O3: Dy3þ (x ¼ 40%) (b–d).
crystalline when compared with the undoped CaTiO3. These results are corroborated by the SEM observations. The strongest diffraction peak of the Zr4þ doped CaTiO3:Dy3þ at 32.8� (2θ) successively shifted toward low angles with increasing Zr4þ dopant concentration. This trend can be mostly attributed to the substitution of the surrounding atoms and to the regrouping of the density states in the CaTiO3 unit cell. Because Zr4þ (0.72 Å) has a larger ionic radius when compared with that of Ti4þ
octahedron, whereas A2þ is located at the center of eight BO6 and co ordinates with twelve O2 , as it shown in Fig. 3(a) [12]. The lattice parameters of the two PDF cards are presented in Table 1. The results in Table 1 indicate that the lattice parameters of the crystal increase with increasing amount of Zr4þ doped into the CaTiO3 crystal lattice. However, the intensity of the diffraction peaks decreased at high Zr4þ concentrations, indicating that the Zr4þ doped samples were less 3
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Fig. 4. Scanning electron microscope images of the samples in different concentration of Zr4þ occupy the Ti4þ(a. no Zr4þ, b. 20% Zr4þ, c. 40% Zr4þ, d. 60% Zr4þ, e. EDS pattern for Ca(Ti1 xZrx)O3: Dy3þ (x ¼ 40%) phosphor).
Pbnm(62)) as the initial model, with zero correction, peak shape, background, and other parameters being preset in the program. A pseudo-Voigt function was selected for peak fitting. The calculated value (YCal) was obtained by adjusting the structural parameters of CaTiO3 (e. g. cell constants, atomic position parameters, thermal parameters, and position occupancy) using multiple iterations and by subsequently per forming least-squares fitting. Finally, the full-spectrum fitting results for Ca(Ti1 xZrxO3:Dy3þ (x ¼ 0, 20%, 40%, 60%) in Fig. 2 were obtained. The correction factors wRp, Rp, and χ 2 were also obtained (Fig. 2). The difference between the line representing the experimental results and that representing the theoretical calculation results is small, indicating that the structure is reasonable. After the refinement was completed, the unit-cell values of the samples were obtained, as presented in Table 2. All the samples exhibit orthorhombic structures, and their unit-cell volumes expanded upon doping with Zr4þ, demonstrating that the Zr4þ with a large ionic radius was successfully doped into the host and induced lattice expansion. In addition, the cell parameters of Ca(Ti0⋅4Zr0.6)O3 substantially changed.
Table 4 eZAF intelligent quantitative results of Ca(Ti1 xZrx)O3:Dy3þ (x ¼ 40%) phosphor. Element
Weight%
Atom%
Net strength
Error %
O Zr Ca Ti Dy
31.66 18.39 26.57 18.13 5.24
60.81 6.19 20.37 11.63 0.99
46.89 134.30 259.60 137.20 10.63
14.28 3.26 4.16 5.77 46.66
(0.61 Å), the partial replacement of Ti4þ with Zr4þ leads to the expan sion and deformation of the crystal lattice of CaTiO3:Dy3þ, which further influences the photoluminescent intensity [24,25]. The crystal size after Zr4þ codoping was calculated, and the corre sponding Zr4þ position in the crystal lattice was confirmed through Rietveld refinement using the GSAS program [26]. The refinement with GSAS is based on CaTiO3 (orthorhombic crystal system, space group 4
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decreased with increasing Zr4þ dopant concentration, and the aggre gation phenomenon became increasingly obvious. In addition, with increasing concentration of Zr4þ, the distribution of grain size broad ened, consistent with the decrease in crystallinity with increasing Zr4þ dopant concentration observed in the XRD analysis. The EDS results indicate that the samples in our study comprised Ca, Ti, Zi, O, and Dy and that no other elements were detected (Fig. 4(e)). The Ca(Ti1 xZrx)O3:Dy3þ (x ¼ 0.40) phosphor eZAF intelligent test using the EDS results are presented in Table 4. The atomic concentra tions of Zr and Ti atoms are 6.19% and 11.63%, respectively, and the Zr/ Ti ratio is 0.32:0.6, which is close to the 0.4:0.6 stoichiometric ratio of the raw materials used during the synthesis. These results indicate that the Zr4þ are almost completely incorporated into the CaTiO3 lattice, which act in cooperation with the XRD analysis results. 3.2. Luminescence properties Fig. 5 denotes the photoluminescence spectra (PL) of the CaTi O3:0.02Dy3þ phosphor. Sharp excitation peaks were observed at 353, 365 and 387 nm form the excitation spectrum, corresponding to the 6 H15/2 → 6P7/2, 6H15/2 → 6P5/2 and 6H15/2 → 4I13/2 transitions of Dy3þ, respectively [28,29]. The intensities of the excitation peaks were observed to be similar at 353, 365 and 387 nm. Therefore, NUV wave lengths of 353, 365 and 387 nm were selected to excite the white phosphors for obtaining the emission spectra. The emission intensity under the excitation at 365 nm was greatest when compared with the intensity of the emission spectra excited at 353 and 387 nm. In the emission spectrum, all the specimens exhibited the following two emission peaks: a yellow one at 578 nm and a blue one at 484 nm, corresponding to the 4F9/2 → 6H13/2 and 4F9/2 → 6H15/2 transitions of Dy3þ, respectively. The combination of these two lights can produce a white light [30,31]. The inset is the chromaticity coordinate diagram of CaTiO3:Dy3þ excited at 365 nm, it shows the point located in the white light area. Fig. 6 depicts the excitation spectrum of Ca(Ti1 xZrx)O3:Dy3þ (0 � x � 60%) in the 320–440 wavelength range with the blue-light emission at 484 nm. The peak shapes of the excitation spectra exhibi ted no difference. However, the peak intensities of the samples sub stantially differed. Specifically, the excitation intensity obviously increased with increasing Zr4þ doping concentration. The highest exci tation intensity was observed for the sample with a Zr4þ concentration of 40% from among these samples. When the Zr4þ concentration exceeded 40%, the excitation intensity decreased. As depicted in Fig. 7(a), the intensity of the two emission peaks increased after the substitution of Zr4þ at the Ti4þ sites, and the law of variation of the emission spectra is consistent with that of the excitation spectrum. The emission intensity gradually increased with increasing Zr4þ. The highest emission peak was achieved when the concentration of Zr4þ was 40%. The increase in emission peak intensity is owed to the expansion of the lattice and the change in the environment of the crystal field [23]. When the Zr4þ concentration exceeded 40%, the emission intensity decreased because of the decrease in crystallinity with increasing Zr4þ concentration. Furthermore, the CaTiO3 crystal contained an inversion center in the case of Dy3þ was in the environment of high symmetry position in the crystal lattice. In this case, the blue light emission corresponding to the magnetic transition was more intense [32]. Fig. 7(b) is a line chart of the emission intensities of the 484 nm blue light and the 578 nm yellow light as a function of the Zr4þ concentration. The results denoted that the blue-light intensity was greater than the yellow light intensity, indi cating that all the phosphors occupied high-symmetry sites in the crystal lattice and that codoping Zr4þ did not transform the high-symmetry position of Dy3þ. Fig. 7(b) also denoted that the emission intensity at 578 nm increased more than that at 484 nm with the increasing amount of incorporated Zr4þ. As depicted in Fig. 7(c), the ratio intensity of blue and yellow emission are closest to 1:1 after 0.4 Zr4þ codoping and
3þ
Fig. 5. PL spectra of CaTiO3:Dy phosphor at room temperature. The inset is the chromaticity coordinates diagram.
Fig. 6. The excitation (0 � x � 60%) phosphors.
spectrum
of
Ca(Ti1-xZrx)O3:Dy3þ
The regulation of the change in the crystal unit-cell parameters is consistent with the standard PDF card presented in Table 1. Table 3 presents the atomic position coordinates of Ca(Ti0⋅6Zr0.4)O3: Dy3þ phodphor, as obtained from the refinement calculations. The re sults denote that the Zr atoms have the same coordinate positions and the same Uiso as the Ti atoms, indicating that the Zr4þ replaced the Ti4þ in the crystal structure. In addition, the occupancy rate of the Zr4þ and Ti4þ was 0.44 and 0.56, respectively, consistent with the original stoi chiometric ratio. The Ca and Dy atoms also occupied the same position in the lattice, indicating that Dy3þ substituted Ca2þ in the crystal lattice; this result was consistent with that obtained by Fei et al. [22]. Table 3 also denotes that the O atoms occupied two different posi tions in the lattice. At one position, eight O2 coordinated with the Ca2þ to form a distorted dodecahedron, whereas six O2 coordinated with a Ti4þ in an octahedral manner at the other position [27]. The crystal structure of the CaTiO3 perovskite presented in Fig. 3(a) can be visual ized in an intuitive manner. The crystal structure of Ca(Ti0⋅6Zr0.4)O3: Dy3þ based on the results of the Rietveld refinement using the GSAS program is presented in Fig. 3(b–d). Fig. 4 presents the SEM images of the samples. All the particles exhibited nanoscale dimensions. The compactness of the sample 5
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Optical Materials 98 (2019) 109446
Fig. 7. (a) The emission spectrum of Ca(Ti1-xZrx)O3:Dy3þ (0 � x � 60%) phosphors, (b) concentration-emission intensity line graph, (c) the intensity ratio of blue emission to yellow emission. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
Fig. 8. Decay curves of Ca(Ti1-XZrX)O3:Dy3þ (0 � x � 60%) phosphors.
meanwhile the intensity of yellow emission increased faster than that of blue. This phenomenon indicated that codoping Zr4þ in the lattice more strongly influences the electric couple transition (4F9/2 → 6H13/2) than magnetic transition (4F9/2 → 6H15/2). The electric dipole transition (578 nm) is a hypersensitive transition, which is a class of transitions strongly influenced by the external conditions around Dy3þ [33]. The results indicated that the addition of Zr4þ to the CaTiO3:Dy3þ host effectively changed the ratio of blue to yellow light, resulting in enhanced luminescence properties of the phosphor. The estimated decay time of the Ca(Ti1 xZrx)O3:Dy3þ (0 � x � 60%) phosphors were calculated, the outcomes are exhibited in Fig. 8. The fluorescence lifetime decay curve was drawn by fitting the second-order
exponential function in Equation (1), and the average fluorescence lifetime (τ) was calculated using Equation (2). y ¼ y0 þ A1 expð
τ¼
A1 τ21 þ A2 τ22 ; A1 τ 1 þ A2 τ 2
x
x0
τ1
Þ þ A2 expð
x
x0
τ2
Þ;
(1) (2)
In the equations, A1 and A2 denote constants, τ1 and τ2 represent the decay times of photoluminescence. The results denote that the lifetime of the Ca(Ti1 xZrx)O3:Dy3þ phosphors varies from 0.552 to 0.545, 0.539, 0.526, 0.522, 0.504, and 0.485 ms for x ¼ 0–60% mol Zr4þ, 6
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4. Conclusions A series of Ca(Ti1 xZrx)O3:Dy3þ samples were prepared using the combustion method fueled by citric acid. The XRD patterns denoted that all the samples exhibited an orthorhombic perovskite-like crystal structure and that the main phase varied from CaTiO3 to CaZrO3 with increasing concentration of Zr4þ doping. The GSAS program was used to analyze the structure of the samples, revealing that the lattice parameter changed after the doping of Zr4þ into the CaTiO3 crystal lattice and that Zr4þ occupied the position of the Ti4þ in the crystal structure. Further more, the SEM observations revealed that the nanometric grains varied with the varying concentration of Zr4þ, and EDS confirmed the elemental composition of the samples. As the dopant, Zr4þ enhanced the PL intensity of CaTiO3:Dy3þ, the optimum concentration of Zr4þ was 40%. Furthermore, Ca(Ti0⋅6Zr0.4)O3:Dy3þ exhibited the lowest color temperature and was the closest to the standard white light from among all the investigated samples. The decay time of the Ca(Ti0⋅6Zr0.4)O3:Dy3þ phosphors under NUV excitation were observed to be suitable for the application of these materials in WLEDs. Declaration of competing interest
Fig. 9. CIE coordinates of the of Ca(Ti1-XZrX)O3:Dy3þ (x ¼ 0, 20%, 40%,60%) phosphors.
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
respectively. These results indicate that the decay time decreases as Zr4þ increases from x ¼ 0 to x ¼ 60%. This decrease in decay time is caused by the high rate of radiative transition from the excited state to the ground state [34,35]. The CIE coordinates of the Ca(Ti1 xZrx)O3:Dy3þ (0 � x � 60%) phosphors are depicted in Fig. 9. When the excitation wavelength was 365 nm, the CIE coordinates of the Ca(Ti1 xZrx)O3:Dy3þ (x ¼ 0, 20%, 40%, 60%) phosphors were determined to be (0.3175, 0.4033), (0.3284, 0.3981), (0.3414, 0.4000), and (0.3322, 0.4016), respectively. All the coordinates of the as-synthesized phosphors correspond well to those of the white-light region (0.33, 0.33). The colonelcy is determined by the color temperature, which is an indicator of the spectral quality of the light source. The lower the color temperature, the warmer will be the yellow light; the larger the color temperature, the colder will be the light. The color temperature of the three samples was calculated using equations (3) and (4) [36]. CCT ¼ 437n3 þ 3061n2 þ 6861n þ 5514:31; n¼
x y
xe ; ye
Acknowledgments The authors gratefully acknowledge the financial support from the Key Scientific and Technological Research and Development Program (Grant number 2017GZ0400), Sichuan Province, PR China. References [1] J. Huang, L. Zhou, Z. Liang, F. Gong, J. Han, R. Wang, Promising red phosphors LaNbO4:Eu3þ, Bi3þ for LED solid-state lighting application, J. Rare Earths 28 (2010) 356–360. [2] P. Liu, J. Yin, X. Mi, L. Zhang, L. Bie, Enhanced photoluminescence of CaTiO3:Eu3þ red phosphors prepared by H3BO3 assisted solid state synthesis, J. Rare Earths 31 (2013) 555–558. [3] F. Liu, Y. Fang, N. Zhang, G. Zhao, Y. Liu, Enhancement of white light emission of Dy3þ:CaTiO3 phosphor by Liþ co-doping, J. Mater. Sci. Mater. Electron. 26 (2015) 3933–3938. [4] J. Zhang, Y. Fan, Z. Chen, J. Wang, P. Zhao, B. Hao, Enhancing the photoluminescence intensity of CaTiO3:Eu3þ red phosphors with magnesium, J. Rare Earths 33 (2015) 1036–1039. [5] Y. Wang, H. Zhang, Q. Wei, C. Su, D. Zhang, Solid state synthesis, tunable luminescence and thermal stability of NaCaBO3:Eu2þ/Mn2þ phosphors, Ceram. Int. 42 (2016) 12422–12426. [6] Q. Tang, K. Qiu, W. Zhang, Y. Shen, J. Wang, Luminescence enhancement of Ca3Sr3(VO4)4:Eu3þ, Sm3þ red-emitting phosphor by charge compensation, Opt. Mater. 75 (2018) 258–266. [7] X. Zhang, M. Gong, Single-phased white-light-emitting NaCaBO3: Ce3þ, Tb3þ, Mn2þ phosphors for LED applications, Dalton Trans. 43 (2014) 2465–2472. [8] P. Yang, X. Yu, X. Xu, T. Jiang, H. Yu, D. Zhou, Z. Yang, Z. Song, J. Qiu, Singlephased CaAl2Si2O8:Tm3þ, Dy3þ white-light phosphors under ultraviolet excitation, J. Solid State Chem. 202 (2013) 143–148. [9] C. Yan, Z. Liu, W. Zhuang, R. Liu, X. Xing, Y. Liu, G. Chen, Y. Li, X. Ma, YScSi4N6C: Ce3þ a broad cyan emitting phosphor to weaken the cyan cavity in full spectrum white light emitting diodes, Inorg. Chem. 56 (2017) 11087–11095. [10] Z.W. Zhang, A.J. Song, M.Z. Ma, X.Y. Zhang, Y. Yue, R.P. Liu, A novel white emission in Ca8MgBi(PO4)7:Dy3þ single-phase full-color phosphor, J. Alloy. Comp. 601 (2014) 231–233. [11] G. Li, Solid state synthesis and luminescence of NaLa(WO4)2:Dy3þ phosphors, J. Mater. Sci. Mater. Electron. 27 (2016) 11012–11016. [12] G.N. Bhargavi, A. Khare, Luminescence studies of perovskite structured titanates: a review, Opt Spectrosc. 118 (2015) 902–917. [13] L. Gracia, J. Andres, V.M. Longo, J.A. Varela, E. Longo, A theoretical study on the photoluminescence of SrTiO3, Chem. Phys. Lett. 493 (2010) 141–146. [14] C. Peng, Z. Hou, C. Zhang, G. Li, H. Lian, Z. Cheng, J. Lin, Synthesis and luminescent properties of CaTiO3: Pr3þ microfibers prepared by electrospinning method, Opt. Express 18 (2010) 7543–7553. [15] Y. Zhu, X. Wang, Y. Zhou, C. Zhao, J. Yuan, Z. Wu, S. Wu, S. Wang, In situ formation of bioactive calcium titanate coatings on titanium screws for medical implants, RSC Adv. 6 (2016) 53182–53187.
(3) (4)
where xe ¼ 0.3320, ye ¼ 0.1858, and x and y represents the CIE coordinates. The color temperatures of the Ca(Ti1 xZrx)O3:Dy3þ (x ¼ 0, 10%, 20%, 30%, 40%, 50%, and 60%) phosphors were 5987.84, 5928.83, 5631.69, 5494.98, 5220.12, 5272.97, and 5507.95, respectively. High color temperatures are bad for the human eye; relatively yellow and soft white light is more suitable for daily application conditions. The results denote that the color temperature is the lowest and most suitable for the human eye in case of phosphor with a Zr4þ doping concentration of x ¼ 40%. The fluorescence QY refers to the ratio of a sample’s generated photoelectron energy to the photoelectron energy absorbed at a particular wavelength under identical conditions. The excitation at 365 nm was used to collect the 355–650 nm spectra of samples and blanks. Further, the difference between the areas of the blanks and samples represents the difference between the photoelectron energy absorbed by the sample and the generated photoelectron energy at a characteristic excitation peak of the Dy3þ at 365 nm and two emission peaks of 484 and 578 nm. The QY of the Ca(Ti0⋅6Zr0.4)O3:Dy3þ phosphor was measured to be 9.9%. 7
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[27] P.j. Zeng, L.p. Yu, Z.x. Qiu, J.l. Zhang, C.y. Rong, C.z. Li, Z.h. Fu, S.x. Lian, Significant enhancement of luminescence intensity of CaTiO3:Eu3þ red phosphor prepared by sol-gel method and co-doped with Bi3þ and Mg2þ, J. Sol. Gel Sci. Technol. 64 (2012) 315–323. [28] Q. Liu, Y. Liu, Y. Ding, Z. Peng, X. Tian, Q. Yu, G. Dong, A white light emitting luminescent material Ba3Y(PO4)3:Dy3þ, Ceram. Int. 40 (2014) 10125–10129. [29] X. Li, L. Guan, M. Sun, H. Liu, Z. Yang, Q. Guo, G. Fu, Luminescent properties of Dy3þ doped SrMoO4 phosphor, J. Lumin. 131 (2011) 1022–1025. [30] J. Sun, X. Zhang, Z. Xia, H. Du, Synthesis and luminescence properties of novel LiSrPO4:Dy3þ phosphor, Mater. Res. Bull. 46 (2011) 2179–2182. [31] Y. Liu, Z. Yang, Q. Yu, X. Li, Y. Yang, P. Li, Luminescence properties of Ba2LiB5O10: Dy3þ phosphor, Mater. Lett. 65 (2011) 1956–1958. [32] M. Yu, J. Lin, Z. Wang, J. Fu, S. Wang, H.J. Zhang, Y.C. Han, Fabrication, patterning, and optical properties of nanocrystalline YVO4: a (A ¼ Eu3þ, Dy3þ, Sm3 þ , Er3þ) phosphor films via sol-gel soft lithography, Chem. Mater. 14 (2002) 2224–2231. [33] H. Chen, P. Loiseau, G. Aka, Optical properties of Dy3þ doped CaYAlO4 crystal, J. Lumin. 199 (2018) 509–515. [34] C.S. Vandana, B.H. Rudramadevi, The effect of Mg2þ concentration on structural and luminescent properties of CaTiO3: Eu3þ phosphor, Ferroelectrics 524 (2018) 168–180. [35] J.S. Kim, Y.H. Park, S.M. Kim, J.C. Choi, H.L. Park, Temperature-dependent emission spectra of M2SiO4 : Eu2þ (M ¼ Ca, Sr, Ba) phosphors for green and greenish white LEDs, Solid State Commun. 133 (2005) 445–448. [36] Y. Li, J. Chen, C. Chen, Tunable correlated color temperature of NaSrPO4 phosphors via Dy3þ and Eu3þ co-doping for warm white light-emitting diodes, Optik 174 (2018) 1–6.
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Optical Materials journal homepage: http://www.elsevier.com/locate/optmat
Crystal structure and luminescence properties of CaTiO3:Dy3þ phosphor co-doped with Zr4þ Qian Yin a, Kehui Qiu b, *, Wentao Zhang a, Xianfei Chen a, Peicong Zhang a, Qinxue Tang a, Min Chen a a b
College of Materials and Chemistry & Chemical Engineering, Chengdu University of Technology, Chengdu, 610059, China Institute of Materials Science and Technology, Chengdu University of Technology, Chengdu, 610059, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Combustion Ca(Ti1 xZrx)O3:Dy3þ WLEDs Structural substitution
A series of Ca(Ti1 xZrx)O3:Dy3þ phosphors were fabricated using a combustion method. The lattice structure, morphological features, and luminescence performances of the specimens were obtained. Rietveld refinement of the XRD patterns confirmed the crystal distortion of CaTiO3 after Zr4þcodoping and the positional occupation of Zr4þ in the CaTiO3 crystal lattice. The photoluminescence spectroscopy results indicated that the fluorescence emission intensity of CaTiO3:Dy3þ was substantially enhanced by Zr4þ codoping and that the optimum doping concentration of Zr4þ was 40% mol. The quantum yield of the Ca(Ti0⋅6Zr0.4)O3:Dy3þ phosphor was measured to be 9.9%, and its fluorescence lifetime decreased with the increasing concentration of Zr4þ doping. Furthermore, the lowest color temperature was observed for Ca(Ti0⋅6Zr0.4)O3:Dy3þ among the investigated phosphors and its color coordinate was the closest to the standard white-light color coordinate. These results suggest that the Ca (Ti0⋅6Zr0.4)O3:Dy3þ phosphor excited by near-ultraviolet light is a potential material for application in white light-emitting diodes.
1. Introduction White light-emitting diodes (WLEDs) are currently regarded as the most extensively used solid-state lighting to replace incandescent and fluorescent lamps. They exhibit numerous advantages compared with incandescent and fluorescent lamps, including lesser environmental impact, longer lifetimes, enhanced safety, and higher efficiency [1–4]. Currently, the combination of a light-emitting diode (LED) chip and phosphor is considered to be the most effective approach for generating white light. For instance, in commercial devices, a blue GaN LED chip is combined with the Y3Al5O12:Ce3þ yellow phosphor [5,6]. However, this approach often causes defects. For example, over time, aging of the chip can decrease the color rendering index and instability of the color temperature because of a lack of red emission. In addition, blue LED chips can emit excessive blue light. Biological and medical researchers have reported that excessive blue light increases the risk of psycholog ical or physiological issues such as depression and sleep disorders [7–9]. Another approach to generate white light is to combine semiconductor chips that emit near-ultraviolet (NUV) light with tricolor phosphors. However, multiphase phosphors reduce the luminous efficiency in
practical applications because of the differences in decay times and the reabsorption of emission colors. A single-composition phosphor that emits white light under NUV excitation and binds to LEDs in a potential solid-state lighting material would overcome these deficiencies [10,11]. Calcium titanate (CaTiO3) is the representative perovskite structure exhibiting a general formula of ABO3, where A is a cation with a large ionic radius like Ca2þ and B is a cation with a smaller radius, mainly Ti4þ or Nb5þ [12]. CaTiO3 presents excellent chemical stability, electrical conductivity, and biocompatibility. Because of these advantages, CaTiO3 is extensively used in high-frequency dielectric MLC ceramic materials, dielectric resonators and luminescent materials [13–15]. Recently, CaTiO3 has been proposed as a luminescent phosphor host material because of its remarkable properties [16–19]. Moreover, the f–f transition of the Dy3þ causes it to produce two emission peaks, located at 578 nm (yellow) and 484 nm (blue) matching with the 4F9/2 → 6H13/2 and 4F9/2 → 6H15/2 transitions of the Dy3þ, respectively, which are excited by NUV [20]. The yellow-light and blue-light can be combined to come into being white light under NUV excitation [21]. Based on the aforementioned advantages, CaTiO3:Dy3þ phosphors have been recently proposed. However, their emission intensity is
* Corresponding author. E-mail address: [email protected] (K. Qiu). https://doi.org/10.1016/j.optmat.2019.109446 Received 4 September 2019; Received in revised form 8 October 2019; Accepted 8 October 2019 Available online 17 October 2019 0925-3467/© 2019 Elsevier B.V. All rights reserved.
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Fig. 1. X-ray diffraction patterns of Ca(Ti1 xZrx)O3:Dy3þ (0 � x � 60%) synthesized at 900 � C.
mixed in a beaker. The mixed solution in the beaker was subsequently stirred using a magnetic at the temperature of 80 � C. After the mixture became clear, anhydrous citric acid was added and the resulting mixture was stirred for approximately 20 min. The previously prepared Ca (NO3)2 and Dy(NO3)3 aqueous solutions were subsequently added into the aforementioned mixture; the resulting mixture was stirred until it formed a buff sol precursor. Finally, the obtained specimen was sintered at 900 � C for 1 h in a high temperature muffle furnace to gain a white powder.
Table 1 The lattice parameters of the PDF cards. Chemical formula
PDF card
Space group
a (Å)
b(Å)
c(Å)
v(Å3)
CaTiO3
82–0229
5.4086
5.4553
7.6782
226.55
CaZrO3
76–2401
Pbnm (62) Pcnm (62)
5.5912
8.0171
5.7616
258.26
inadequate to satisfy the application requirements. Therefore, re searchers have been attempting to increase the emission intensity of the CaTiO3:Dy3þ phosphors through charge compensation, crystal distor tion and structural substitution [22,23]. Because the Zr4þ has a coor dination number of six and can form an octahedron with six O2 , similar to the octahedral structure of the Ti4þ, we proposed that codoping Zr4þ into the CaTiO3:Dy3þ phosphor would enhance its luminescence properties. Herein, a series of Ca(Ti1 xZrx)O3:Dy3þ phosphors were synthesized using a combustion method for the first time. Further, the changes in the crystal parameters of these samples with variations in the concentration of Zr4þ were investigated and the position of Zr4þ in the CaTiO3 crystal structure was discussed. In addition, the effects of Zr4þ on the photo luminescence characteristics and crystal lattice structure of these Zr4þ doped phosphors were analyzed.
2.2. Sample characterization Phosphors were structurally characterized by X-ray diffraction (XRD) with Cu Kα radiation (λ ¼ 1.5418 Å, 40 mA, 40 kV Philips X’Pert Pro MPD). The morphology of the as-synthesized samples was observed via scanning electron microscopy (SEM) (model JEX-100CX, magnifi cation 20,000 � , voltage 10 kV). The compositional analysis of the samples was conducted using energy-dispersive X-ray spectroscopy (EDS). Further, the photoluminescence performances of the specimen were tested by a spectrophotometer (PL Hitachi F-4600) assembled with a Xe lamp (150-W) operated at 500 V. The quantum yield (QY) of the phosphor was obtained through FSL 980 (Edinburgh). All the samples were measured at ambient temperature. 3. Results and discussion
2. Experimental
3.1. Formation and morphology
2.1. Sample synthesis
Fig. 1 depicts the XRD of Ca(Ti1 xZrx)O3:Dy3þ (0 � x � 60%) phos phors acquired at 900 � C using the combustion method. The patterns of all the samples indicated an orthorhombic perovskite-like structure. However, the crystal parameters changed as the chemical composition of the samples was varied. The diffraction peaks of the obtained speci mens completely fitted with the standard diffraction site and intensity of CaTiO3 (PDF card 82–0229). Specifically, as x was increased from 0 to 60%, the synthesized matrix material gradually shifted from CaTiO3 to CaZrO3 (PDF card 76–2401). All these standard PDF cards refer to ABO3 perovskite-type oxides in which A denotes the Ca atoms and B denotes the Ti/Zr atoms. One B4þ and six O2 coordinate to form a BO6
3þ
A series of Ca(Ti1 xZrx)O3:Dy (x ¼ 0, 10%, 20%, 30%, 40%, 50%, 60%) phosphors were synthesized via the combustion method. The raw materials were Ca(NO3)2⋅4H2O (AR grade), Ti(OC4H9)4 (AR grade), (CH2OH)2 (AR grade), Zr(NO3)4⋅5H2O (AR grade), HNO3 (AR grade), absolute ethanol (AR grade), anhydrous citric acid (AR grade), and highpurity Dy2O3 powder (99.9%). First, Dy2O3 was dissolved in a mixture of 0.2 mL of HNO3 and 3 mL of distilled water. Then adding the Ca (NO3)2⋅4H2O in the mixed solution. Subsequently, 3.5 mL of Ti(OC4H9)4 and 6.2 mL of (CH2OH)2 were measured using a graduated cylinder and 2
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Fig. 2. The Rietveld profile fits the diffraction data of Ca(Ti1 xZrx)O3:Dy3þ (x ¼ 0, 20%, 40%, 60%). Table 2 The lattice parameter of the Ca(Ti1-xZrx)O3:Dy3þ(x ¼ 0, 20%, 40%, 60%) after refinement. Chemical formula 3þ
CaTiO3: Dy Ca(Ti0⋅8Zr0.2)O3: Dy3þ Ca(Ti0⋅6Zr0.4)O3: Dy3þ Ca(Ti0⋅4Zr0.6)O3: Dy3þ
a (Å)
b(Å)
c(Å)
v(Å3)
5.3902 5.4368 5.4982 5.5895
5.4437 5.5239 5.6394 7.7934
7.6052 7.7206 7.8075 5.1579
223.1593 231.8692 242.0832 224.6828
Table 3 Coordinates of each atom in Ca(Ti1-xZrx)O3: Dy3þ (x ¼ 40%) phosphor and its occupancy rate. Atoms
x
y
z
Occupancies
Uiso
Ca O1 O2 Ti Zr Dy
0.026718 0.079816 0.215062 0.000000 0.000000 0.026718
0.526757 0.031634 0.300832 0.000000 0.000000 0.526757
0.250000 0.250000 0.043354 0.000000 0.000000 0.250000
0.985581 1.000000 1.000000 0.560678 0.439322 0.014418
0.025109 0.039563 0.049757 0.020123 0.020123 0.025109
Fig. 3. The crystal structure of the standard CaTiO3 (a) and the Ca(Ti1 xZrx)O3: Dy3þ (x ¼ 40%) (b–d).
crystalline when compared with the undoped CaTiO3. These results are corroborated by the SEM observations. The strongest diffraction peak of the Zr4þ doped CaTiO3:Dy3þ at 32.8� (2θ) successively shifted toward low angles with increasing Zr4þ dopant concentration. This trend can be mostly attributed to the substitution of the surrounding atoms and to the regrouping of the density states in the CaTiO3 unit cell. Because Zr4þ (0.72 Å) has a larger ionic radius when compared with that of Ti4þ
octahedron, whereas A2þ is located at the center of eight BO6 and co ordinates with twelve O2 , as it shown in Fig. 3(a) [12]. The lattice parameters of the two PDF cards are presented in Table 1. The results in Table 1 indicate that the lattice parameters of the crystal increase with increasing amount of Zr4þ doped into the CaTiO3 crystal lattice. However, the intensity of the diffraction peaks decreased at high Zr4þ concentrations, indicating that the Zr4þ doped samples were less 3
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Fig. 4. Scanning electron microscope images of the samples in different concentration of Zr4þ occupy the Ti4þ(a. no Zr4þ, b. 20% Zr4þ, c. 40% Zr4þ, d. 60% Zr4þ, e. EDS pattern for Ca(Ti1 xZrx)O3: Dy3þ (x ¼ 40%) phosphor).
Pbnm(62)) as the initial model, with zero correction, peak shape, background, and other parameters being preset in the program. A pseudo-Voigt function was selected for peak fitting. The calculated value (YCal) was obtained by adjusting the structural parameters of CaTiO3 (e. g. cell constants, atomic position parameters, thermal parameters, and position occupancy) using multiple iterations and by subsequently per forming least-squares fitting. Finally, the full-spectrum fitting results for Ca(Ti1 xZrxO3:Dy3þ (x ¼ 0, 20%, 40%, 60%) in Fig. 2 were obtained. The correction factors wRp, Rp, and χ 2 were also obtained (Fig. 2). The difference between the line representing the experimental results and that representing the theoretical calculation results is small, indicating that the structure is reasonable. After the refinement was completed, the unit-cell values of the samples were obtained, as presented in Table 2. All the samples exhibit orthorhombic structures, and their unit-cell volumes expanded upon doping with Zr4þ, demonstrating that the Zr4þ with a large ionic radius was successfully doped into the host and induced lattice expansion. In addition, the cell parameters of Ca(Ti0⋅4Zr0.6)O3 substantially changed.
Table 4 eZAF intelligent quantitative results of Ca(Ti1 xZrx)O3:Dy3þ (x ¼ 40%) phosphor. Element
Weight%
Atom%
Net strength
Error %
O Zr Ca Ti Dy
31.66 18.39 26.57 18.13 5.24
60.81 6.19 20.37 11.63 0.99
46.89 134.30 259.60 137.20 10.63
14.28 3.26 4.16 5.77 46.66
(0.61 Å), the partial replacement of Ti4þ with Zr4þ leads to the expan sion and deformation of the crystal lattice of CaTiO3:Dy3þ, which further influences the photoluminescent intensity [24,25]. The crystal size after Zr4þ codoping was calculated, and the corre sponding Zr4þ position in the crystal lattice was confirmed through Rietveld refinement using the GSAS program [26]. The refinement with GSAS is based on CaTiO3 (orthorhombic crystal system, space group 4
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decreased with increasing Zr4þ dopant concentration, and the aggre gation phenomenon became increasingly obvious. In addition, with increasing concentration of Zr4þ, the distribution of grain size broad ened, consistent with the decrease in crystallinity with increasing Zr4þ dopant concentration observed in the XRD analysis. The EDS results indicate that the samples in our study comprised Ca, Ti, Zi, O, and Dy and that no other elements were detected (Fig. 4(e)). The Ca(Ti1 xZrx)O3:Dy3þ (x ¼ 0.40) phosphor eZAF intelligent test using the EDS results are presented in Table 4. The atomic concentra tions of Zr and Ti atoms are 6.19% and 11.63%, respectively, and the Zr/ Ti ratio is 0.32:0.6, which is close to the 0.4:0.6 stoichiometric ratio of the raw materials used during the synthesis. These results indicate that the Zr4þ are almost completely incorporated into the CaTiO3 lattice, which act in cooperation with the XRD analysis results. 3.2. Luminescence properties Fig. 5 denotes the photoluminescence spectra (PL) of the CaTi O3:0.02Dy3þ phosphor. Sharp excitation peaks were observed at 353, 365 and 387 nm form the excitation spectrum, corresponding to the 6 H15/2 → 6P7/2, 6H15/2 → 6P5/2 and 6H15/2 → 4I13/2 transitions of Dy3þ, respectively [28,29]. The intensities of the excitation peaks were observed to be similar at 353, 365 and 387 nm. Therefore, NUV wave lengths of 353, 365 and 387 nm were selected to excite the white phosphors for obtaining the emission spectra. The emission intensity under the excitation at 365 nm was greatest when compared with the intensity of the emission spectra excited at 353 and 387 nm. In the emission spectrum, all the specimens exhibited the following two emission peaks: a yellow one at 578 nm and a blue one at 484 nm, corresponding to the 4F9/2 → 6H13/2 and 4F9/2 → 6H15/2 transitions of Dy3þ, respectively. The combination of these two lights can produce a white light [30,31]. The inset is the chromaticity coordinate diagram of CaTiO3:Dy3þ excited at 365 nm, it shows the point located in the white light area. Fig. 6 depicts the excitation spectrum of Ca(Ti1 xZrx)O3:Dy3þ (0 � x � 60%) in the 320–440 wavelength range with the blue-light emission at 484 nm. The peak shapes of the excitation spectra exhibi ted no difference. However, the peak intensities of the samples sub stantially differed. Specifically, the excitation intensity obviously increased with increasing Zr4þ doping concentration. The highest exci tation intensity was observed for the sample with a Zr4þ concentration of 40% from among these samples. When the Zr4þ concentration exceeded 40%, the excitation intensity decreased. As depicted in Fig. 7(a), the intensity of the two emission peaks increased after the substitution of Zr4þ at the Ti4þ sites, and the law of variation of the emission spectra is consistent with that of the excitation spectrum. The emission intensity gradually increased with increasing Zr4þ. The highest emission peak was achieved when the concentration of Zr4þ was 40%. The increase in emission peak intensity is owed to the expansion of the lattice and the change in the environment of the crystal field [23]. When the Zr4þ concentration exceeded 40%, the emission intensity decreased because of the decrease in crystallinity with increasing Zr4þ concentration. Furthermore, the CaTiO3 crystal contained an inversion center in the case of Dy3þ was in the environment of high symmetry position in the crystal lattice. In this case, the blue light emission corresponding to the magnetic transition was more intense [32]. Fig. 7(b) is a line chart of the emission intensities of the 484 nm blue light and the 578 nm yellow light as a function of the Zr4þ concentration. The results denoted that the blue-light intensity was greater than the yellow light intensity, indi cating that all the phosphors occupied high-symmetry sites in the crystal lattice and that codoping Zr4þ did not transform the high-symmetry position of Dy3þ. Fig. 7(b) also denoted that the emission intensity at 578 nm increased more than that at 484 nm with the increasing amount of incorporated Zr4þ. As depicted in Fig. 7(c), the ratio intensity of blue and yellow emission are closest to 1:1 after 0.4 Zr4þ codoping and
3þ
Fig. 5. PL spectra of CaTiO3:Dy phosphor at room temperature. The inset is the chromaticity coordinates diagram.
Fig. 6. The excitation (0 � x � 60%) phosphors.
spectrum
of
Ca(Ti1-xZrx)O3:Dy3þ
The regulation of the change in the crystal unit-cell parameters is consistent with the standard PDF card presented in Table 1. Table 3 presents the atomic position coordinates of Ca(Ti0⋅6Zr0.4)O3: Dy3þ phodphor, as obtained from the refinement calculations. The re sults denote that the Zr atoms have the same coordinate positions and the same Uiso as the Ti atoms, indicating that the Zr4þ replaced the Ti4þ in the crystal structure. In addition, the occupancy rate of the Zr4þ and Ti4þ was 0.44 and 0.56, respectively, consistent with the original stoi chiometric ratio. The Ca and Dy atoms also occupied the same position in the lattice, indicating that Dy3þ substituted Ca2þ in the crystal lattice; this result was consistent with that obtained by Fei et al. [22]. Table 3 also denotes that the O atoms occupied two different posi tions in the lattice. At one position, eight O2 coordinated with the Ca2þ to form a distorted dodecahedron, whereas six O2 coordinated with a Ti4þ in an octahedral manner at the other position [27]. The crystal structure of the CaTiO3 perovskite presented in Fig. 3(a) can be visual ized in an intuitive manner. The crystal structure of Ca(Ti0⋅6Zr0.4)O3: Dy3þ based on the results of the Rietveld refinement using the GSAS program is presented in Fig. 3(b–d). Fig. 4 presents the SEM images of the samples. All the particles exhibited nanoscale dimensions. The compactness of the sample 5
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Optical Materials 98 (2019) 109446
Fig. 7. (a) The emission spectrum of Ca(Ti1-xZrx)O3:Dy3þ (0 � x � 60%) phosphors, (b) concentration-emission intensity line graph, (c) the intensity ratio of blue emission to yellow emission. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
Fig. 8. Decay curves of Ca(Ti1-XZrX)O3:Dy3þ (0 � x � 60%) phosphors.
meanwhile the intensity of yellow emission increased faster than that of blue. This phenomenon indicated that codoping Zr4þ in the lattice more strongly influences the electric couple transition (4F9/2 → 6H13/2) than magnetic transition (4F9/2 → 6H15/2). The electric dipole transition (578 nm) is a hypersensitive transition, which is a class of transitions strongly influenced by the external conditions around Dy3þ [33]. The results indicated that the addition of Zr4þ to the CaTiO3:Dy3þ host effectively changed the ratio of blue to yellow light, resulting in enhanced luminescence properties of the phosphor. The estimated decay time of the Ca(Ti1 xZrx)O3:Dy3þ (0 � x � 60%) phosphors were calculated, the outcomes are exhibited in Fig. 8. The fluorescence lifetime decay curve was drawn by fitting the second-order
exponential function in Equation (1), and the average fluorescence lifetime (τ) was calculated using Equation (2). y ¼ y0 þ A1 expð
τ¼
A1 τ21 þ A2 τ22 ; A1 τ 1 þ A2 τ 2
x
x0
τ1
Þ þ A2 expð
x
x0
τ2
Þ;
(1) (2)
In the equations, A1 and A2 denote constants, τ1 and τ2 represent the decay times of photoluminescence. The results denote that the lifetime of the Ca(Ti1 xZrx)O3:Dy3þ phosphors varies from 0.552 to 0.545, 0.539, 0.526, 0.522, 0.504, and 0.485 ms for x ¼ 0–60% mol Zr4þ, 6
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4. Conclusions A series of Ca(Ti1 xZrx)O3:Dy3þ samples were prepared using the combustion method fueled by citric acid. The XRD patterns denoted that all the samples exhibited an orthorhombic perovskite-like crystal structure and that the main phase varied from CaTiO3 to CaZrO3 with increasing concentration of Zr4þ doping. The GSAS program was used to analyze the structure of the samples, revealing that the lattice parameter changed after the doping of Zr4þ into the CaTiO3 crystal lattice and that Zr4þ occupied the position of the Ti4þ in the crystal structure. Further more, the SEM observations revealed that the nanometric grains varied with the varying concentration of Zr4þ, and EDS confirmed the elemental composition of the samples. As the dopant, Zr4þ enhanced the PL intensity of CaTiO3:Dy3þ, the optimum concentration of Zr4þ was 40%. Furthermore, Ca(Ti0⋅6Zr0.4)O3:Dy3þ exhibited the lowest color temperature and was the closest to the standard white light from among all the investigated samples. The decay time of the Ca(Ti0⋅6Zr0.4)O3:Dy3þ phosphors under NUV excitation were observed to be suitable for the application of these materials in WLEDs. Declaration of competing interest
Fig. 9. CIE coordinates of the of Ca(Ti1-XZrX)O3:Dy3þ (x ¼ 0, 20%, 40%,60%) phosphors.
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
respectively. These results indicate that the decay time decreases as Zr4þ increases from x ¼ 0 to x ¼ 60%. This decrease in decay time is caused by the high rate of radiative transition from the excited state to the ground state [34,35]. The CIE coordinates of the Ca(Ti1 xZrx)O3:Dy3þ (0 � x � 60%) phosphors are depicted in Fig. 9. When the excitation wavelength was 365 nm, the CIE coordinates of the Ca(Ti1 xZrx)O3:Dy3þ (x ¼ 0, 20%, 40%, 60%) phosphors were determined to be (0.3175, 0.4033), (0.3284, 0.3981), (0.3414, 0.4000), and (0.3322, 0.4016), respectively. All the coordinates of the as-synthesized phosphors correspond well to those of the white-light region (0.33, 0.33). The colonelcy is determined by the color temperature, which is an indicator of the spectral quality of the light source. The lower the color temperature, the warmer will be the yellow light; the larger the color temperature, the colder will be the light. The color temperature of the three samples was calculated using equations (3) and (4) [36]. CCT ¼ 437n3 þ 3061n2 þ 6861n þ 5514:31; n¼
x y
xe ; ye
Acknowledgments The authors gratefully acknowledge the financial support from the Key Scientific and Technological Research and Development Program (Grant number 2017GZ0400), Sichuan Province, PR China. References [1] J. Huang, L. Zhou, Z. Liang, F. Gong, J. Han, R. Wang, Promising red phosphors LaNbO4:Eu3þ, Bi3þ for LED solid-state lighting application, J. Rare Earths 28 (2010) 356–360. [2] P. Liu, J. Yin, X. Mi, L. Zhang, L. Bie, Enhanced photoluminescence of CaTiO3:Eu3þ red phosphors prepared by H3BO3 assisted solid state synthesis, J. Rare Earths 31 (2013) 555–558. [3] F. Liu, Y. Fang, N. Zhang, G. Zhao, Y. Liu, Enhancement of white light emission of Dy3þ:CaTiO3 phosphor by Liþ co-doping, J. Mater. Sci. Mater. Electron. 26 (2015) 3933–3938. [4] J. Zhang, Y. Fan, Z. Chen, J. Wang, P. Zhao, B. Hao, Enhancing the photoluminescence intensity of CaTiO3:Eu3þ red phosphors with magnesium, J. Rare Earths 33 (2015) 1036–1039. [5] Y. Wang, H. Zhang, Q. Wei, C. Su, D. Zhang, Solid state synthesis, tunable luminescence and thermal stability of NaCaBO3:Eu2þ/Mn2þ phosphors, Ceram. Int. 42 (2016) 12422–12426. [6] Q. Tang, K. Qiu, W. Zhang, Y. Shen, J. Wang, Luminescence enhancement of Ca3Sr3(VO4)4:Eu3þ, Sm3þ red-emitting phosphor by charge compensation, Opt. Mater. 75 (2018) 258–266. [7] X. Zhang, M. Gong, Single-phased white-light-emitting NaCaBO3: Ce3þ, Tb3þ, Mn2þ phosphors for LED applications, Dalton Trans. 43 (2014) 2465–2472. [8] P. Yang, X. Yu, X. Xu, T. Jiang, H. Yu, D. Zhou, Z. Yang, Z. Song, J. Qiu, Singlephased CaAl2Si2O8:Tm3þ, Dy3þ white-light phosphors under ultraviolet excitation, J. Solid State Chem. 202 (2013) 143–148. [9] C. Yan, Z. Liu, W. Zhuang, R. Liu, X. Xing, Y. Liu, G. Chen, Y. Li, X. Ma, YScSi4N6C: Ce3þ a broad cyan emitting phosphor to weaken the cyan cavity in full spectrum white light emitting diodes, Inorg. Chem. 56 (2017) 11087–11095. [10] Z.W. Zhang, A.J. Song, M.Z. Ma, X.Y. Zhang, Y. Yue, R.P. Liu, A novel white emission in Ca8MgBi(PO4)7:Dy3þ single-phase full-color phosphor, J. Alloy. Comp. 601 (2014) 231–233. [11] G. Li, Solid state synthesis and luminescence of NaLa(WO4)2:Dy3þ phosphors, J. Mater. Sci. Mater. Electron. 27 (2016) 11012–11016. [12] G.N. Bhargavi, A. Khare, Luminescence studies of perovskite structured titanates: a review, Opt Spectrosc. 118 (2015) 902–917. [13] L. Gracia, J. Andres, V.M. Longo, J.A. Varela, E. Longo, A theoretical study on the photoluminescence of SrTiO3, Chem. Phys. Lett. 493 (2010) 141–146. [14] C. Peng, Z. Hou, C. Zhang, G. Li, H. Lian, Z. Cheng, J. Lin, Synthesis and luminescent properties of CaTiO3: Pr3þ microfibers prepared by electrospinning method, Opt. Express 18 (2010) 7543–7553. [15] Y. Zhu, X. Wang, Y. Zhou, C. Zhao, J. Yuan, Z. Wu, S. Wu, S. Wang, In situ formation of bioactive calcium titanate coatings on titanium screws for medical implants, RSC Adv. 6 (2016) 53182–53187.
(3) (4)
where xe ¼ 0.3320, ye ¼ 0.1858, and x and y represents the CIE coordinates. The color temperatures of the Ca(Ti1 xZrx)O3:Dy3þ (x ¼ 0, 10%, 20%, 30%, 40%, 50%, and 60%) phosphors were 5987.84, 5928.83, 5631.69, 5494.98, 5220.12, 5272.97, and 5507.95, respectively. High color temperatures are bad for the human eye; relatively yellow and soft white light is more suitable for daily application conditions. The results denote that the color temperature is the lowest and most suitable for the human eye in case of phosphor with a Zr4þ doping concentration of x ¼ 40%. The fluorescence QY refers to the ratio of a sample’s generated photoelectron energy to the photoelectron energy absorbed at a particular wavelength under identical conditions. The excitation at 365 nm was used to collect the 355–650 nm spectra of samples and blanks. Further, the difference between the areas of the blanks and samples represents the difference between the photoelectron energy absorbed by the sample and the generated photoelectron energy at a characteristic excitation peak of the Dy3þ at 365 nm and two emission peaks of 484 and 578 nm. The QY of the Ca(Ti0⋅6Zr0.4)O3:Dy3þ phosphor was measured to be 9.9%. 7
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Optical Materials 98 (2019) 109446
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