Site Occupancy Studies and Luminescence Properties of Emission

2 days ago - The structure, site occupancies, and photoluminescence (PL) properties of Ce3+- or Eu2+-doped Ca9La(PO4)7 phosphors were investigated ...
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Site Occupancy Studies and Luminescence Properties of Emission Tunable Phosphors Ca9La(PO4)7:Re (Re = Ce3+, Eu2+) Mubiao Xie,* Haifeng Wei, and Weijie Wu School of Chemistry and Chemical Engineering, Lingnan Normal University, Zhanjiang 524048, China

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S Supporting Information *

ABSTRACT: The structure, site occupancies, and photoluminescence (PL) properties of Ce3+- or Eu2+-doped Ca9La(PO4)7 phosphors were investigated in this work. The powder X-ray diffraction (XRD) data and Rietveld refinement results showed that the phosphors are pure phases. The vacuum ultraviolet-ultraviolet− visible (VUV-UV−vis) spectra and decay curves confirmed that the Ce3+ and Eu2+ ions in Ca9La(PO4)7 occupied two crystal sites. Additionally, the tunable color can be obtained by adjusting doping concentration and altering temperature. The fabricated w-LED lamp based on Eu2+-doped Ca9La(PO4)7 phosphor gave properties with a color rendering index (CRI) of 85.26, CIE chromaticity coordinates of (0.358, 0.307), and color temperature (CCT) of 4329 K.

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from ultraviolet to red. Xia21 research group have made a systematic summary of this work in their review papers recently. Because the crystal field can be adjustable, one growing interest of β-Ca3(PO4)2-related phosphors is exploring the w-LED phosphors through the Ce3+-Mn2+, Ce3+-Eu2+, Eu2+-Mn2+, Ce3+-Mn2+ codoping system, such as the reported Ca9Ln(PO4)7 (Ln = Al, Rare earth) based phosphors, which have been extensively studied as color-tunable phosphors for w-LED.22−27 However, detailed studies of site occupancy of Ce3+ or Eu2+ in materials Ca9La(PO4)7:Ce3+ (CLP:Ce3+) and Ca9La(PO4)7:Eu2+ (CLP: Eu2+) have not been reported so far. When Ce3+ or Eu2+ are introduced into Ca9La(PO4)7, at least two different Ce3+ or Eu2+ emissions are expected due to the rich cation sites in Ca9La(PO4)7. In this work, the photoluminescence properties of Ce3+-doped and Eu2+-doped Ca9La(PO4)7 phosphors are systematically studied. The Ce3+ and Eu2+ ions have been found to enter two different sites in Ca9La(PO4)7, and the thermal quenching and decay behaviors of two different Ce3+ or Eu2+ centers are studied. The tunable emission of Eu2+ has been realized by adjusting the Eu2+ doping contents and the temperature. The results show that the phosphors CLP: Eu2+ can be considered as the cyan luminescent component of w-LEDs and temperature sensor materials.

INTRODUCTION Rare-earth-ion Ce3+ or Eu2+-doped luminescent materials are considered to be interesting materials because of their various applications.1,2 One of these properties is designed to be a tunable-emission luminescent material for lighting, displaying, and detecting.3−10 At present, there are three main methods to achieve tunable emission in the Ce3+- or Eu2+-doped phosphors: (1) Adjusting the host structure with the substitution of cation or anion to achieve tunable emission, according to the theory that the outer 5d orbit of Ce3+ or Eu2+ ions is strongly influenced by coordination environments of the crystal;11 (2) Co-doping with two or more ions (Ce3+, Eu2+, Mn2+, Tb3+) in a host;12 (3) Singly doping Ce3+ or Eu2+ ion into a lattice with multiple sites to produces multiple emitting centers.13,14 It is believed that the tunable emitting obtained through the third way is more attractive, because such system only contains one activator and one host. Phosphate-based phosphors are considered to be good candidates for novel phosphors because of their high chemical or physical stabilities. Among so many phosphate-based hosts, the whitlockite-type compound β-Ca3(PO4)2 has attracted much attention recently, which structure contains three P sites and six Ca sites with space group R3c.15 One of the interests for scientist is the rich metal sites in β-Ca3(PO4)2, which can make the original structure produce series of compounds with β-Ca3(PO4)2 type structure, such as Ca9Ln(PO4)7 (Ln = Cr, Al, Bi and Rare earth),16,17 Ca8MgR(PO4)7:Eu3+ (R = Rare earth),18 Ca10A(PO4)7 (A = alkali metal).1,19,20 Accordingly, the β-Ca3(PO4)2 and its variations are considered as a promising host for luminescent materials and biomaterials recently. As single-doped luminescent materials, the βCa3(PO4)2-type compounds are usually doped with Ce3+ or Eu2+ ions to obtain phosphors with emission wavelength range © XXXX American Chemical Society

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EXPERIMENTAL SECTION

Materials and Synthesis. A high-temperature, solid-state reaction method was utilized to prepared the phosphors Ca9La(PO4)7:Ce3+ and Ca9La(PO4)7:Eu2+ with CaCO3 (analytical reagent, A.R.), NH4H2PO4 (A.R.), La2O3 (99.99%), CeO2 (99.99%) and Received: September 13, 2018

A

DOI: 10.1021/acs.inorgchem.8b02597 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. (i) XRD patterns of the representative phosphors and the standard data of Ca9Y(PO4)7 as reference (JCPDS No. 46−0402); (ii) simulated crystal structure of Ca9La(PO4)7; (iii) coordination environment of the Ca2+ and La3+ sites.

Committee for Powder Diffraction Standard (JCPDS) files 46−0402 [Ca9Y(PO4)7], it can be seen that no impurity peaks were found in the diffraction curves of all samples. Figure 1ii, iii give the crystal structure and the coordination environment of cationic sites in Ca9La(PO4)7, respectively. Figure 1ii show that The structure of β-Ca3(PO4)2-type compound Ca9La(PO4)7 is isostructural to the Ca9Y(PO4)7. In Figure 1iii, four types of coordination environments of cationic sites can be found in Ca9La(PO4)7, which are labeled as Ca(1), Ca(2), Ca(3) and La. The coordinated numbers of Ca(1), Ca(2), Ca(3) and La are 8, 8, 9 and 6, respectively.26 Figure 2 shows the Rietveld refinement of the representative samples Ca9La(PO4)7, Ca9La(PO4)7:0.10Ce3+, and Ca9La(PO4)7:0.10Eu2+, and the Rietveld refinement results are shown in Table 1. The calculated profiles are in good agreement with the experimental data, and the obtained reliability factors Rp and Rwp indicate that the refine results are reliable, which further verifies the phase purity of the samples. When Ce3+ or Eu2+ are doped into the host, the cell parameters a, b, c, and cell volume become larger. It is due to the larger effective ionic radii of Ce3+ and Eu2+ than of La3+ and Ca2+ with the same coordination number. Luminescence of Ce3+ in Ca9La(PO4)7. The PL spectra at different wavelength excitations of CLP:0.10Ce3+ recorded on a deuterium lamp at 10 K are shown in Figure 3. Two emission bands centered at ∼350 nm and ∼440 nm are observed clearly under short-wavelength excitations (within 202−325 nm).

Eu2O3 (99.99%) as raw materials. First, stoichiometric amounts of raw materials were mixed and ground thoroughly in an agate mortar. The mixtures were then put into the corundum crucible, and reduced at 1473 K for 4 h under a reduction atmosphere (H2:N2 = 1:4). Finally, the samples were cooled to room temperature and reground into white powder. Characterization Method. The X-ray powder diffraction (XRD) data were collected on a D8 Advance diffractometer (Bruker Corporation) with Cu Kα radiation (λ = 0.15418 nm), operating at 40 kV and 40 mA at room temperature (RT). The data for the Rietveld refinement were collected over a 2θ range from 10 to 130° with an step size of 0.02°. The academic software TOPAS was used to perform the Rietveld refinement, and the crystal structures were obtained by software Diamond 3 using the CIF file of Ca9La(PO4)7. The photoluminescence (PL), photoluminescence excitation (PLE) spectra, luminescence decay curves and quantum efficiency were carried out by an FLS 920 spectrometer (Edinburgh Instruments Ltd., U.K.) with a integrating sphere and temperature-controlling device. A 450 W xenon lamp and a nF900 flash lamp were used as the excitation source for the steady-state and transient-state, respectively. The vacuum ultraviolet-ultraviolet excitation spectra were measured by using a spectrometer with a vacuum monochromator, and a 150 W Deuterium lamp was used as an excitation source.

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RESULTS AND DISCUSSION XRD Patterns and Crystal Structure Refinements. The XRD patterns of representative phosphors CLP:0.01Ce3+ (a), CLP:0.10Ce3+ (b), CLP:0.01Eu2+ (c), and CLP:0.10Eu2+ (d) are displayed in Figure 1(i). By comparing with the Joint B

DOI: 10.1021/acs.inorgchem.8b02597 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 3. Emission spectra of the sample CLP:0.10Ce3+ at different excitation wavelengths at 10 K (Deuterium lamp as excitation source).

Figure 2. Experimental (black crosses) and calculated (red line) XRD patterns of the Rietveld refinement of Ca9La(PO4)7, Ca9La(PO4)7:0.10Ce3+ , and Ca9La(PO 4)7:0.10Eu2+. The blue lines represent the difference between experimental and calculated data, and the vertical magenta lines mark the Bragg reflections.

Figure 4. Gaussian functions for emission spectrum of the sample CLP:0.10Ce3+ under 225 nm excitation at 100 K (Xenon lamp as excitation source).

440 nm emissions do not come from the same Ce3+ center because energy difference is much greater than 2000 cm−1 here. The Ce3+ centers with shorter emission wavelength is labeled as Ce(1)3+, while that with longer emission wavelength is marked as Ce(2)3+. The PL spectra under different wavelength excitations of CLP:0.10Ce3+ recorded on xenon lamp at 10K are shown in Figure S1, which shows similar emission characters. According to the discussion above, Ce3+ ions with emission centers would have four emission bands due to the transitions 5d−2F5/2 and 5d−2F7/2. However, the

Except for 160 and 172 nm, with the excitation wavelength increasing, the short-wavelength emission (350 nm) increases slowly, and reaches maximum at the 245 nm excitation, whereas the long-wavelength emission (440 nm) disappears gradually. This phenomenon is mainly due to the existence of different Ce3+ centers that are related to different excitation light. It is known that the Ce3+ emission energy difference from the lowest 5d excited states to the 2F5/2 and 2F7/2 ground states is 2000 cm−1. Therefore, it can be concluded that the 350 and

Table 1. Refined Results and Structural Parameters of Ca9La(PO4)7, Ca9La(PO4)7:0.10Ce3+ and Ca9La(PO4)7:0.10Eu2+ samples

Rp (%)

Rwp (%)

a (Å)

b (Å)

c (Å)

Ca9La(PO4)7 Ca9La(PO4)7:0.10Ce3+ Ca9La(PO4)7:0.10Eu2+

6.12 6.67 6.62

8.39 9.45 9.11

10.4657 10.4711 10.4727

10.4657 10.4711 10.4727

37.5189 37.5360 37.5656

C

DOI: 10.1021/acs.inorgchem.8b02597 Inorg. Chem. XXXX, XXX, XXX−XXX

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nm, peak 3), and ∼2.70 eV (∼460 nm, peak 4). The energy differences between peak 1 and peak 2, peak 3 and peak 4 are calculated as 2134 and 1790 cm−1, respectively. Such energy differences are in close proximity to the usual energy difference (∼2000 cm−1) from 5d−2F5/2 and 5d−2F7/2 transitions, which further confirms the fact that two Ce3+ centers exist in Ca9La(PO4)7. The VUV excitation spectra is good method to analyze the site occupancy. Figure 5 gives the VUV excitation spectra of the sample CLP:0.10Ce3+ by monitoring different emissions with Deuterium lamp at 10K. As seen from curves a, b, and c in Figure 5, the excitation curves by monitoring Ce(1) 3+ emissions (340, 360, and 380 nm) show almost the same spectral shape. Five evident excitation bands which are marked B1−B5 are observed above 200 nm. The band with a maximum value of ∼315 nm belongs to the transition of the Ce (1)3+ center from the 4f ground state to the lowest excited state (5d1). As seen from curve d, the excitation spectrum show a different characteristic with curves a, b and c when monitored at a longer wavelength emission (400 nm). It is seen that the excitation band at ∼315 nm has red-shift a little, and the bands B4 and B5 nearly disappear. When the monitoring wavelength is 440 nm, which is mainly corresponding to Ce(2)3+ centers, another group of excitation bands (marked as C1 to C5) in curve e are observed. The lowest excited 5d energy level locates at ∼325 nm, and the bands C4 and C5 at ∼200 and ∼215 nm are dominant. Thus, it is reasonable that the bands C4 and C5 should be the 5d excitation bands of Ce(2)3+. Similar results can be seen in the excitation spectrum measured by different emissions on xenon lamp in the UV range, as shown in Figure S2. Additionally, the high-energy absorption peak at 165 nm (marked as H) is assigned to the host-related absorptions according to the reported phosphors Ca9Y(PO4)7:Ce and Ca10M(PO4)7: Ce3+ (M = Li, Na, K).28−30 The attribution of excitation and emission peaks together with the Stokes shift values for Ce(1)3+ and Ce(2)3+ are listed in Table 2. Dorenbos has concluded that the decrease in the 5d energy for different rare-earth ions in one lattice is nearly changeless, and the value for Ce3+ ion is significant to predict the 5d energy for other rare-earth ions.31,32 The crystal field depression D(A) for Ce3+ ion can be calculated according to eq 1:

Figure 5. VUV excitation spectra of the sample CLP:0.10Ce3+ by monitoring different emissions with Deuterium lamp at 10 K.

Table 2. Luminescent Properties of Two Different Ce3+ Centers in Ca9La(PO4)7 Ce centers

characteristics

Ce(1)

5d states (nm) emission bands (nm) Stokes shift (×103 cm−1) 5d states (nm) emission bands (nm) Stokes shift (×103 cm−1)

Ce(2)

value 315, 330, 3.58 325, 430, 9.03

293, 265, 250, 222 355 305, 277, 215, 200 460

D(A) = E(Ce, free) − E(Ce, A)

(1)

where E(Ce, free) represents the 4f−5d transition energies of the free Ce3+ ions, which is a constant of 49.34 × 103 cm−1, and E(Ce, A) represents the lowest 4f−5d transition energies of Ce3+ ions in a defined host A. According to the excitation spectrum analysis above, the calculated E(Ce, A) values is 31.75 × 103 cm−1 (315 nm) for Ce(1)3+ and 30.77 × 103 cm−1 (325 nm) for Ce(2), respectively. Hence, we obtain the calculated D(A) values 17.59 × 103 cm−1 (Ce(1)3+) and 29.41 × 103 cm−1 (Ce(2)3+). According to analysis, the energy levels of Ce(1)3+ and Ce(2)3+ in Ca9La(PO4)7 is presented in Figure S3. The effective ionic radii is 1.12 Å for Ca2+ (eight coordination), 1.18 Å for Ca2+ (nine coordination), 1.03 Å for La3+ (six coordination), 1.08 Å for Ce3+ (six coordination), 1.14 Å for Ce3+ (eight coordination), and 1.20 Å Ce3+ (nine coordination), respectively. Therefore, it can be predicted that the Ce3+ ions would occupy the different Ca2+ and La3+ sites in the host structure according to the effective ionic radii and

Figure 6. Emission spectra (λex = 225 nm) of sample CLP:0.10Ce3+ from 10 to 300 K (Xenon lamp as excitation source). The inset shows temperature-dependent luminescence intensities of Ce(1)3+ at 340 nm and Ce(2)3+ at 430 nm, respectively.

emission bands from 5d−2F5/2 and 5d−2F7/2 are usually not separable, especially for the PL spectra measured at RT. The Gaussian fitting is a common way to find out the definite emission position of Ce3+. The Gaussian fitting result based on the emission spectrum under 225 nm excitation recorded with Deuterium lamp at 100 K is shown in Figure 4. It is found that the emission spectrum can be fitted well by a sum of four Gaussian functions, with maximum peaks at ∼3.76 eV (∼330 nm, peak 1), ∼ 3.49 eV(∼355 nm, peak 2), ∼ 2.88 eV (∼430 D

DOI: 10.1021/acs.inorgchem.8b02597 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 7. Excitation ((a) λem = 420, recorded by the deuterium lamp; λem = 425 nm, recorded by the xenon lamp; (b) λem = 495, recorded by the deuterium lamp; λem = 496 nm, recorded by the xenon lamp) and emission spectra ((c) λex = 280 nm; (d) λex = 330 nm) of CLP:0.10Eu2+ recorded at 10 K.

Figure 8. Emission spectra of CLP:0.10Eu2+ excited by different wavelengths at RT (Xenon lamp as excitation source).

Figure 9. (a) Emission spectra of CLP:xEu2+ phosphors on the concentration of dopant Eu2+ ions under excitation at 280 nm (Xenon lamp as excitation source); (b) normalized emission spectra of CLP:xEu2+ samples; (c)corresponding CIE chromaticity coordinates of the samples.

ionic charge matching. When we further consider the bonddistance difference between La−O and Ca−O, it is reasonable to assign Ce(1)3+ as being related to the Ca2+ sites and the Ce(2)3+ as corresponding to the La3+ sites in Ca9La(PO4)7, because the Ce3+ ions in the smaller La3+ sites will have lower 5d energy. One question still puzzling us is whether Ce(1)3+ ions enter into eight-coordinated or nine-coordinated Ca2+ sites. Van Uitert has built a following experiential eq 2 to estimate the emission peak position by the coordination environment.33 ÄÅ ÉÑ 1/ V ÅÅÅ ÑÑÑ i y V nEar j z ÑÑ E = Q ÅÅÅ1 − jj zz 10 ÅÅ 80 ÑÑÑÑÖ k4{ (2) ÅÇ

where E (cm−1) represents the Ce3+ or Eu2+ emission peak in one site; Q (cm−1) represents the energy of the lower d-band edge for the free Ce3+ or Eu2+ ion (Q = 50000 cm−1 for Ce3+ and 34000 cm−1 for Eu2+); V is the valence of the activator (V = 3 for Ce3+ and 2 for Eu2+); Ea is the electron affinity of the atoms that form anions, which is different in the host with different anion, and here Ea is determined to be 3.12 eV approximately.32 n is the coordination number of Ce3+ or Eu2+, and r is the radius of the cation Ca2+ or La3+. Accordingly, the E for Ce3+ ions in eight-coordinated Ca2+ sites (n = 8, r = 1.12 Å) is calculated to be 29669 cm−1(337 nm), whereas that in night-coordinated Ca2+ sites (n = 9, r = 1.18 Å) is 32450 cm−1(308 nm) and that in six-coordinated La2+ sites is 23263 E

DOI: 10.1021/acs.inorgchem.8b02597 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 10. (a) Emission spectra (λex = 300 nm) of CLP:0.10Eu2+ sample from 10 to 350 K (Xenon lamp as excitation source); (b) normalized emission spectra of CLP:0.10Eu2+ samples; (c) corresponding CIE chromaticity coordinates of the samples. The insets are the photos of the fabricated LED devices:(A) lamp (NUV chip + CLP:0.10Eu2+) in a nonworking state; (B) lamp (NUV chip + CLP:0.10Eu2+) driven by 100 mA; lamp (NUV chip + CLP:0.10Eu2+ + Y2O3:Eu3+) driven by 100 mA.

cm−1 (430 nm). The calculated E values 337 and 430 nm match well with the measured emission peaks of Ce(1)3+ and Ce(2)3+, indicating that the Ce(1)3+ ions with 337 nm emission are originated from Ce3+ in eight-coordinated Ca(1) or Ca(2) sites, whereas the Ce(2)3+ ions with 430 nm emission belongs to the six-coordinated La2+ sites. Influence of Temperature on Two Ce3+ Emissions. Figure 6 shows the PL spectra of the sample CLP:0.10Ce3+ recorded by the xenon lamp upon 225 nm excitation from 10K to 300 K. By comparing with the spectrum recorded by the deuterium lamp under 223 nm excitation at 10 K in Figure 3, it is found that the relative intensity of the Ce(1)3+ (∼350 nm) and Ce(2)3+ (∼440 nm) emission bands in Figure 3 and Figure 6 are not the same. This is due to a different response using different excitation sources and instruments from the instrument. Seen from Figure 5, it can be predicted that Ce(1)3+ and Ce(2)3+ centers can be excited simultaneously under 225 nm excitation. As we expected, both Ce(1)3+ and Ce(2)3+ emissions are observed in Figure 6 at all the temperature range. At low temperature (for example 10 K), the emission intensity of Ce(2)3+ is higher than that of Ce(1)3+. With increasing temperature, the emission intensities of Ce(1)3+ and Ce(2)3+ decrease simultaneously. Furthermore, the intensity decreasing speed of Ce(2)3+ is much faster than that of Ce(1)3+, especially from 10 to 200 K, as clearly shown in the temperature dependent curves in the inset of Figure 6. Such difference is mainly attributed to the larger Stokes of Ce(2)3+. Luminescence of Eu2+ in Ca9La(PO4)7. Figure 7 presents the PLE and PL spectra of CLP:0.10Eu2+, which are recorded at 10K. Figure 7a,b display the excitation spectra by monitoring different emissions from the VUV to the UV region, recorded by the Xenon and Deuterium lamp, respectively. Above 200 nm, two weak bands corresponding to the host absorption of PO43− are observed in both Figure 7a and Figure 7b, which is in accordance with that in CLP:0.10Ce3+ as discussed in Figure 5. However, by monitoring different emissions, the excitation curves show

different spectrum characteristics in the wavelength range 220 to 400 nm. When 420 nm is monitored, the maximum peak is located at ∼280 nm, whereas the maximum peak is at ∼300 and ∼330 nm when 495 nm is monitored. Clearly, these bands contribute to the transitions of Eu2+ from 4f to 5d states. Figure 7c, d presents emission spectra excited by 280 and 330 nm, respectively. Both bands centered at ∼425 and ∼495 nm can be seen, but the relative intensities are different. Under 280 nm excitation, the dominant emission band is ∼425 nm, while that is ∼495 nm under 330 nm excitation. As two types of Ce3+ have been assigned in CLP:0.10Ce3+ as discussed above, it is possible to attribute these two bands to be the 5d → 4f transition of Eu2+ ions from two sites. Accordingly, the Eu2+ ions with shorter emission wavelength (425 nm) is marked as Eu(1)2+, and that with longer emission wavelength (495 nm) is marked as Eu(2)2+. The PL spectra at different wavelength excitations of CLP:0.10Eu2+ at 10K are shown in Figure 8. It is observed that the dominant emission gradually change from Eu(1)2+ to Eu(2)2+ with the excitation wavelength changing from 280 to 330 nm. The relationship between the emission bands and excitation wavelength is in good agreement with that Figure 7a, b. That is, 280 nm excitation light mainly excites Eu(1)2+, while 330 nm excitation light excites more efficiently. Hence, we assign 280 nm to the lowest 5d state of Eu(1)2+, and 330 nm belongs to the lowest 5d state of Eu(2)2+. According to the above eq 2, the calculated E are 23263 cm−1(430 nm) for eight-coordinated Ca sites and 20202 cm−1(495 nm) for six-coordinated La site, respectively. The calculated results fit well with the measurement emission peaks in Figure 7. Hence, it is reasonable to assign that Eu(1)2+ emission is originated from Eu2+ ions in the Ca(1) or Ca(2) site, and Eu(2)2+ emission at longer wavelength side belongs to the Eu2+ centers in La sites. The energy levels of Eu(1)2+ and Eu(2)2+ centers in Ca9La(PO4)7 is presented in Figure S4 according to the analysis. Influence of Doping Concentration on Two Eu2+ Emissions. Figure 9a gives the PL spectra of CLP:xEu2+ (x F

DOI: 10.1021/acs.inorgchem.8b02597 Inorg. Chem. XXXX, XXX, XXX−XXX

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regularly with temperature increasing. When the Eu(1)2+ emissions are normalized, as seen in Figure 10b, it is observed clearly that the relative intensity of Eu(2)2+ first increases, and decreases after 150 K. The same phenomena also occur when excited by 280 and 330 nm, as seen in the Figures S5 and S6. As depicted in Figure 10c, the chromaticity diagram shows that the corresponding chromaticity points present an inverted V shape in the cyan region with increase of temperature. This result suggests CLP:0.10Eu2+ may be a potential application in temperature sensors. The optimal phosphor CLP:0.10Eu2+ is selected to measure the quantum efficiency, and the quantum efficiency of CLP:0.10Eu2+ are measured to be 27.62%. Furthermore, a LED device is fabricated combining with the CLP:0.10Eu2+ phosphor and a NUV 365 nm chip, and the photo of the device is shown in the inset A of the Figure 10c. As the LED lamp is driven by 100 mA current, it emits intense cyan light (inset B). To evaluate the potential application of CLP:0.10Eu2+ in w-LED, the mix phosphors CLP:0.10Eu2+ and commercial red-emitting phosphor Y2 O3 :Eu3+ are combined with 365 nm NUV chip to abstain w-LED device. The w-LED lamp is achieved as shown in the inset C of Figure 10c. The w-LED lamp presents properties with color rendering index Ra = 85.26, CIE color coordinates of (0.358, 0.307), and color temperature (CCT) of 4329 K in the white light region. Decay Behaviors of Different Ce3+ and Eu2+ Centers. The luminescence lifetime measurement is considered to be a good means to determine the luminescence centers in one host. The decay curves of the Ce(1)3+, Ce(2)3+, Eu(1)2+, and Eu(2)2+ are depicted in Figure 11. It is found that the luminescence lifetime changes greatly for different Ce3+ or Eu2+ centers. For CLP:0.10Ce3+ sample, the decay curve of Ce(1)3+ (λex = 295 nm, λem = 330 nm) shows single exponential characteristic, and the decay time is calculated to be 0.30 ns. But the decay curve of Ce(2)3+ (λex = 325 nm, λem = 450 nm) is nonsingle exponential with a slower average decay time (0.69 ns), which is mainly due to energy transfer from Ce(1)3+ to Ce(2)3+ centers. For CLP:0.10Eu2+ sample, the decay curve of Eu(1)2+ (λex = 280 nm, λem = 420 nm) is found to be nearly exponential (τ = 20 μs), whereas the decay curve of Eu(2)2+ (λex = 330 nm, λem = 495 nm) presents threeorder decay characteristics (τ1 = 195 ns; τ2 = 1.18 μs; τ3 = 16.7 μs). It verifies the existence of two Eu2+ luminescence centers and the energy transfer process.

Figure 11. (a) Decay curves of the Ce(1)3+ (λex = 295 nm, λem = 330 nm) and Ce(2)3+ (λex = 325 nm, λem = 450 nm); (b) decay curves of the Eu(1)2+ (λex = 280 nm, λem = 420 nm) and Eu(2)2+ (λex = 330 nm, λem = 495 nm).

= 0.01, 0.05, 0.10, 0.20) upon 280 nm excitation as a function of Eu2+ concentration (x). With the increasing of x, the relative intensities of Eu(1)2+ and Eu(2)2+ change a lot. As seen in Figure 9b (the height of 430 nm peaks are normalized), the relative intensity of Eu(2)2+ emission at the longer-wavelength side at about 495 nm increases with increasing Eu 2+ concentration. This phenomena may relate to two factors: the influence of the different Eu2+ centers and energy transfer between the two Eu2+ centers. The combination of site occupancies and energy transfer cause the Eu2+ emission shift toward the longer wavelength side, which makes the emitting color of CLP:xEu2+ phosphors shift from blue to green by adjusting doping concentrations (seen Figure 9c). Influence of Temperature on Two Eu2+ Emissions and LED Fabrication. Figure 10 presents the PL spectra of CLP:0.10Eu2+ excited by 300 nm light at different temperatures from 10 to 350 K. In Figure 10a, It is observed that both Eu(1)2+ and Eu(2)2+ emission decrease with increasing of temperature. The Eu(1)2+ intensities quench until 250 K, while the intensities of Eu(2)2+ do not quench until 350 K. Hence, the relative intensities between Eu(1)2+ and Eu(2)2+ change

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CONCLUSIONS The phosphors CLP:Ce3+ and CLP:Eu2+ have been synthesized through a traditional high-temperature solid-state method. The results of XRD and Rietveld refinements show that the as-synthesized CLP:Ce3+ and CLP:Eu2+ samples are single phase. The luminescence properties of two types of Ce3+ and Eu2+ centers have been studied through PL spectra and lifetime curves in detail. It is concluded that the Ce(1)3+ emission (∼360 nm) and Eu(1)2+ emission (∼425 nm) are considered to be related with eight-coordinated Ca sites, while the Ce(2)3+ emission (∼440 nm) and Eu(2)2+ emission (∼495 nm) are corresponding to six-coordinated La site. The emitting color of Eu2+-doped phosphors can be tuned by adjusting the concentration of Eu2+. The properties of emission spectra changing with temperature indicate that the emission of Ce(2)3+ and Eu(2)2+ have a faster thermal quenching. Accordingly, the emitting color and chromaticity coordinates are related to temperature regularly, which suggest a potential G

DOI: 10.1021/acs.inorgchem.8b02597 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

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application in temperature sensors. The w-LED lamp combined the mix phosphors CLP:0.10Eu2+ and Y2O3:Eu3+ with 365 nm NUV chip shows Ra = 85.26, CCT = 4329 K and CIE color coordinates of (0.358,0.307), which indicate that the material CLP:0.10Eu2+ has potential applications in the NUV w-LED field as a cyan component.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b02597. PL spectra of the sample CLP:0.10Ce3+ at different excitation wavelengths recorded on xenon lamp at 10K (Figure S1); PLE spectra measured by different emissions on xenon lamp in the UV range (Figure S2); energy levels of Ce(1) and Ce(2) centers in Ca9La(PO4)7 (Figure S3); energy levels of Eu(1) and Eu(2) centers in Ca9La(PO4)7 (Figure S4); PL spectra (λex = 280 nm) of CLP:0.10Eu2+ sample from 10 to 350 K (Figure S5) (Xenon lamp as excitation source); PL spectra (λex = 330 nm) of CLP:0.10Eu2+ sample from 10 to 350 K (Figure S6) (Xenon lamp as excitation source) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: 86-759-3183245. Fax: 86759-3183510 (M.X.). ORCID

Mubiao Xie: 0000-0002-1907-4339 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The author thanks the professor Pieter Dorenbos and Erik van der Kolk at Delft University of Technology for providing experiment conditions for VUV PL measurement. The work is financially supported by Research Group of Rare Earth Resource Exploiting and Luminescent Materials (2017KCXTD022), National Natural Science Foundation of China (Grant 21401165), Natural Science Foundation of Guangdong Province (Grant 2014A030307040), and Training Program for Excellent Youth Teachers in Universities of Guangdong Province (YQ2015110).

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REFERENCES

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DOI: 10.1021/acs.inorgchem.8b02597 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.8b02597 Inorg. Chem. XXXX, XXX, XXX−XXX