Tunable Yellow-Red Photoluminescence and Persistent Afterglow in

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Tunable Yellow-Red Photoluminescence and Persistent Afterglow in Phosphors Ca4LaO(BO3)3:Eu3+ and Ca4EuO(BO3)3 Zhen Chen,† Yuexiao Pan,*,† Luqing Xi,† Ran Pang,§ Shaoming Huang,† and Guokui Liu*,‡ †

Key Laboratory of Carbon Materials of Zhejiang Province, College of Chemistry and Materials Engineering, Wenzhou University, Wenzhou 325035, P. R. China ‡ Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, Illinois 60439, United States § State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China S Supporting Information *

ABSTRACT: In most Eu3+ activated phosphors, only red luminescence from the 5D0 is obtainable, and efficiency is limited by concentration quenching. Herein we report a new phosphor of Ca4LaO(BO3)3:Eu3+ (CLBO:Eu) with advanced photoluminescence properties. The yellow luminescence emitted from the 5D1,2 states is not thermally quenched at room temperature. The relative intensities of the yellow and red emission bands depend strongly on the Eu3+ doping concentration. More importantly, concentration quenching of Eu3+ photoluminescence is absent in this phosphor, and the stoichiometric compound of Ca4EuO(BO3)3 emits stronger luminescence than the Eu3+ doped compounds of CLBO:Eu; it is three times stronger than that of a commercial red phosphor of Y2O3:Eu3+. Another beneficial phenomenon is that ligand-to-metal charge transfer (CT) transitions occur in the long UV region with the lowest charge transfer band (CTB) stretched down to about 3.67 eV (∼330 nm). The CT transitions significantly enhance Eu3+ excitation, and thus result in stronger photoluminescence and promote trapping of excitons for persistent afterglow emission. Along with structure characterization, optical spectra and luminescence dynamics measured under various conditions as a function of Eu3+ doping, temperature, and excitation wavelength are analyzed for a fundamental understanding of electronic interactions and for potential applications. emit red luminescence with dominant transitions 5D0 → 7F1 at 590 nm and 5D0 → 7F2 at 610 nm. So far, only a few Eu3+ doped phosphors show yellow-orange luminescence with short wavelength (12 h after excitation.7 LLP luminescence materials that can be activated by blue light with persistent luminescence in the millisecond range are found in applications for alternating-current lightemitting diode (ac-LED) to avoid adverse lighting flicker during each ac cycle.8,9 The green LLP phosphor SrAl2O4:Eu2+,Dy3+ discovered by Yamamoto in 1977 is one of the most utilized LLP phosphors in various displays.10 MAl2O4:Eu2+,Nd3+ (M = Ca, Sr, Ba) are commercially available, and their LLP luminescence lasts longer than 24 h after excitation. In the identified blue LLP phosphors, luminescence of Eu2+ and Dy3+ doped silicates Sr3MgSi3O8 and Sr2MgSi2O7 can last as long as 10 h.11,12 In the case of the tricolor white LLP luminescence materials, a promising red emission with high chemical stability and luminescence efficiency would be desirable although some good candidates for green and blue phosphors have been welldeveloped. Thus, the development of a good red LLP phosphor is a key technology for achieving a tricolor white LLP. The major commercial red LLP phosphors are Eu2+ doped MS (M = Ca, Sr) and Y2O2S:Eu3+, and they emit a red LLP luminescence at night for longer than 12 h.13−15 However, these sulfides are sensitive to moisture that results in a much shorter serving lifetime than blue/green LLP phosphors. Furthermore, sulfide host lattice has low chemical stability and environmental unfriendly characteristics. Some other red LLP phosphors such as Pr3+ doped calcium titanates have been reported but their LLP luminescence properties including efficiency and persistent duration need to be further improved to meet the requirements for practical application.16−18 Many efforts are still devoted to the discovery of a novel red LLP phosphor that has higher chemical stability and luminescence efficiency than sulfides. A crucial challenge is to develop efficient Eu3+ ion doped red phosphors with both high color purity and chemical stability by a facile method. Borates are often chosen as the host lattice of phosphors for that purpose due to their remarkable ability to be incorporated by rare earth ions and ease of processing. However, there was little information available on LLP luminescence of Eu3+ doped borates. Li’s group reported a sol−gel process technique to obtain red Eu3+ doped materials Ca4REO(BO3)3 (RE = La, Y, and Gd). Only normal red emissions that originated from the 5 D0 energy level are observed, which might be due to the sample that was chosen for measurement of photoluminescence having a high concentration of Eu3+ that is larger than 3 mol % of RE3+.19 In the present work, we obtained a novel orange-red phosphor CLBO:Eu via a combustion method, and its color is tunable by modulating Eu3+ concentration. The codoping of Er3+ and Dy3+ can prolong the LLP duration which is significant for practical applications. These findings open up new avenues for the exploration of novel red LLP phosphors with outstanding performances.

3. RESULTS AND DISCUSSION 3.1. Phase and Composition Identification of CLBO:Eu. To investigate the crystallization temperature of CLBO, the ground mixtures were annealed at 900, 1000, 1100, and 1200 °C, respectively. The samples sintered at 900 and 1000 °C include impurity phases. The XRD patterns in Figure S1a reveal that the well-crystallized CLBO could be obtained at a sintering temperature above 1100 °C. All of the diffraction peaks can be indexed to pure CLBO phase which is in good agreement with the standard data of CLBO (JCPDS 52-0621). The crystal structure information on the CLBO:0.2%Eu3+ sample was determined by the Rietveld refinement method as shown in Figure S2. The obtained profile factors are Rp = 5.4% and Rwp = 4.46%, indicating a well fitting quality. The refined lattice parameters of monoclinic CLBO are a = 12.866 Å, b = 16.086 Å, c = 3.626 Å, V = 467.42 Å3, α = γ = 90°, β = 101°, and space group C1m1 (No. 8), which are in agreement with the values in the literatures.20,21 Figure S1b shows the XRD patterns of Ca4La1−xO(BO3)3:xEu3+ with x value varying from 0.01 to 1.0. Diffraction peaks of the corresponding samples coincide well with the standard data of CLBO, and no obvious signal from impurities is observed. The results indicate that all obtained samples are single phase and evident structure change would not occur even when all La3+ ions in CLBO are substituted with Eu3+ ions, which is due to the close ionic radii and the same electric charge of La3+ and Eu3+ ions. The enlarged view on the XRD patterns between 19.6° and 20.5° is shown in Figure S3. The diffraction peaks of representative samples show slight and regular deviation to a larger 2θ angle with the increase of Eu3+ doping concentrations owing to lanthanide contractions. Figure S4a shows SEM images of as-prepared CLBO:Eu. The powder particles are about 50 μm in diameter, and holes are obvious in the particles, due to the functions of gas (such as NO2, CO2, and NH3) produced by combustion of urea.22 The

2. EXPERIMENTIAL DETAILS 2.1. Synthesis. Series of Eu3+ doped Ca4LaO(BO3)3 phosphors were synthesized by a combustion method in air. All starting materials of analytic grade (nitrates Ca(NO3)2 La(NO)3, Eu(NO)3, boric acid H3BO3, and urea) were dissolved in ion-free water and mixed thoroughly in stoichiometric ratios. Subsequently, the homogeneous mixture was transferred into an alumina crucible and sintered at B

DOI: 10.1021/acs.inorgchem.6b01786 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 1. (a) Powder diffuse reflectance spectra of undoped and Eu3+ doped CLBO. (b) Excitation spectra of CLBO:0.2%Eu3+ monitored at 537, 554, 585, and 610 nm, respectively. (c) Emission spectra and (d) CIE chromaticity coordinates of CLBO:0.2%Eu3+ (excited at 266, 331, 395, 462, and 610 nm, respectively).

Figure 2. (a) Emission spectra (excited at 331 nm), (b) excitation spectra (monitored at 583 nm), (c) decay curves (excited at 266 nm and monitored at 610 nm), and (d) CIE chromaticity coordinates of phosphors Ca4La1−xO(BO3)3:xEu3+ (x = 0.2−3%). Photos of samples (e) CLBO:0.2%Eu3+ and (f) CLBO:3.0%Eu3+ under excitation of UV light.

the CLBO host, which is assigned to the host absorption. The band gap of the host CLBO is estimated to be about 5.63 eV (220 nm). With Eu3+ substitution, the absorption band becomes more remarkable, and a strong absorption band at about 270 nm emerges, which is attributed to charge transfer interactions between O2− and dopants Eu3+. The attributions of

EDS analysis (Figure S4b) of the samples shows that the composition constitutes desired compound CLBO:Eu. 3.2. Photoluminescence Properties of CLBO:Eu. Figure 1a depicts the diffuse reflectance spectra of CLBO and CLBO:0.2%Eu3+. A broad absorption band in the range 200− 400 nm with a maximum at about 220 nm is clearly observed in C

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Figure 3. (a) Powder diffuse reflectance spectra of Ca4La1−xO(BO3)3:xEu3+ (x = 0.002−1.0). (b) EDS of sample Ca4La1−xO(BO3)3:xEu3+ with x = 1.0 (inset: SEM of the sample). (c) Emission spectra (excited at 266 nm) of Ca4La1−xO(BO3)3:xEu3+ (x = 0.03−1.0).

The corresponding Commission Internationalede I’Eclairage (CIE) chromaticity coordinates of the CLBO:0.2%Eu3+ sample under different wavelength excitation, calculated from corresponding emission spectra, are exhibited in Figure 1d. The CIE coordinates of the luminescence could be tuned from yellow, to orange, to red with varying excitation wavelengths. Furthermore, Eu3+ emissions from higher energy levels are more effectively excited by CTBs (at 266 and 331 nm) than 4f → 4f transitions (at 395 and 462 nm). 3.3. Influence of Eu3+ Concentration on the Photoluminescence Properties of CLBO:Eu. Blasse stated that the appearance of these transitions from higher energy levels of Eu3+ depends upon the host lattice (phonon frequency as well as the crystal structure) and the doping concentration of Eu3+.23 Thus, the influence of Eu3+ concentration on the photoluminescence properties of CLBO:Eu has been investigated in this work. It is observed that the color of luminescence and the spectral features of CLBO:Eu are strongly dependent on the Eu3+ concentration doped in CLBO as shown in Figure 2. The emission spectra of CLBO:Eu samples with Eu3+ concentration 0.2−3.0 mol % includes peaks in the region covering from 500 to 650 nm. The ratio of emissions attributed to transitions 5D1,2 → 7F0,1,2 (between 440 and 560 nm) to 5D0 → 7F0,1,2 (between 570 and 650 nm) excited at 266 and 331 nm decreases with increase of Eu3+ concentration as observed in Figure 2a and Figure S6. The cross-relaxation occurring between neighboring Eu3+ ions such as 5D1 + 7F0 → 5D0 + 7F3 facilitates quenching of these emissions from higher energy levels 5D1,2.3,5 Furthermore, Figure 2b shows that the CTB2 at 331 nm also decreases with Eu3+ content. Similar changes in the emission (excited at 266 nm) and excitation (monitored at 610 and 537 nm) spectra of phosphors CLBO:Eu are exhibited in Figure S7. The results indicate that the green-yellow emissions of Eu3+ originated from higher energy levels 5D1,2 to the ground states

all bands (or peaks) present in the excitation and emission spectra of CLBO:0.2%Eu3+ are displayed in Figure S5a,b, respectively. The corresponding energy level diagram and the possible optical transitions which are involved in the PL spectra of CLBO:0.2%Eu3+ are depicted in Figure S5c. Excitation spectra of CLBO:0.2%Eu3+ monitored at varying wavelengths as shown in Figure 1b exhibit a strong and broad band centered at 266 nm that is dominantly attributed to a charge transition band (CTB1) caused by the interaction of Eu3+ and O2−. The narrow absorption peaks between 350 and 500 nm are due to the internal 4f → 4f transitions of Eu3+. The excitation spectrum monitored at 610 nm shows typical CTB1 at 266 nm and 4f → 4f transitions of Eu3+ at 395 and 462 nm. A new broad band centered at 331 nm is observed in the excitation spectra monitored at 537, 554, and 585 nm, which probably results from the other charge transition band (CTB2).1−5 As exhibited in Figure S5a, the absorption peaks at 395, 413, and 462 nm are, respectively, attributed to transitions 7F0 to 5L0, 5 D3, 5D2 of Eu3+ according to schematic representation of energy level splitting of Eu3+ ions (in Figure S5c).1−5 In Figure 1c, it is observed that the emission spectra are varying with different excitation wavelengths. Typical red emissions from 5D0 with a maximum at 610 nm are observed in the emission spectra excited at 395 and 462 nm. Under 266 nm excitation, quite plentiful emission peaks covering from blue to red are obtained in the emission spectra with excitation of CTB1 at 266 nm and CTB2 at 331 nm. Figure S5b exhibits detailed attributions of emission peaks excited at 266 nm. The emission peaks in the blue-green-yellow region originate from transitions of Eu3+ from higher energy levels 5D1,2 to the ground states. The emissions located at 577, 585 (593), and 610 (623 nm) are, respectively, attributed to the transitions of Eu3+ from 5D0 to 7F0−2 as denoted in Figure S5c. D

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octahedrons.21 The critical distance between the Eu3+ ions in CEBO is estimated using the equation given by Blasse:24

are dominant in the samples with lower Eu3+ concentration, and these emissions are more efficiently excited by CTB2 at 331 nm than by CTB1 at 266 nm or 4f → 4f transitions 7F0 → 5 L0, 5D3, 5D2 of Eu3+.1−5 The orange-red emissions of Eu3+ (in the region between 570 and 650 nm) that originated from 5D0 to ground states are more efficiently excited by 4f → 4f transitions of Eu3+. As observed in Figure 2c, the decay time of 610 nm emission (excited at 266 nm) decreases with Eu3+ concentration increasing which is due to exchange radiation between luminescence centers.1−5 The CIE coordinates and the luminescence color of samples CLBO:xEu3+ (x = 0.2−3%) depicted in Figure 2d−f vary from yellow to red. Under excitation of UV light, it is observed that the luminescence colors of the phosphors CLBO:0.2%Eu3+ and CLBO:3.0%Eu3+ are yellow and orange, respectively. As shown in Figure 3a, with further increase of Eu3+ concentration, the absorption peaks at 395, 462, and 523 nm are observed in diffuse reflectance spectra of Ca4La1−xO(BO3)3:xEu3+ when x = 1, i.e., Ca4EuO(BO3)3 (CEBO), which is attributed to 4f → 4f transitions of Eu3+ ions. Figure 3b displays EDS and morphology (inset) of CEBO. It is observed in EDS that only signals of calcium (Ca), europium (Eu), boron (B), and oxygen (O) elements could be detected in the as-obtained sample. The XRD (in Figure S1b) and morphology (in Figure 3b, inset) of CEBO are quite similar to those of CLBO. The features of these photoluminescence spectra of Eu3+ are similar when the Eu3+ concentration is higher than 3 mol % as shown in Figure S8. The red emissions that are attributed to transitions 5D0 → 7F0,1,2 are dominant when it is excited by CTB1 at 266 nm, but the CTB2 at 331 nm is not observed in the excitation spectra monitored at 610 nm. The three-dimensional graph of concentration dependence of photoluminescence efficiency (the integrated emission intensity in the wavelength range of 500−650 nm was taken) for CLBO:Eu with Eu3+ concentration higher than 3 mol % has been shown in Figure 3c. The integrated luminescence intensity of CLBO:xEu3+ is linearly enhanced with Eu3+ doping concentration increasing from 3% to 100% of the La3+ site. On the other hand, as shown in Figure S9, all of the decay curves of emissions of phosphors Ca4La1−xO(BO3)3:xEu3+ monitored at different wavelengths decrease with Eu3+ content from x = 3%, 50%, and up to 100%. This is due to the increase of energy transfer and nonradiative relaxation induced by the interaction between Eu3+ ions. For Eu3+ in many hosts, along with shortening of the luminescence decay time, the integrated luminescence intensity decreases after a maximum as Eu3+ concentration increases up to a typical value of 10%. This phenomenon is commonly known as concentration quenching. However, in the CEBO system we studied, such a concentration quenching is absent, although the decay time of the Eu3+ luminescence indeed decreases as the doping concentration increases. Because the integrated luminescence intensity is proportional to the product of the Eu3+ density (concentration) and the decay time, the absence of the concentration quenching suggests that the intensity loss induced by decay time shortening does not offset the intensity gain from the concentration increase. This result is owed to the large distance between Eu3+ ions at the site of La3+ in the CLBO lattice. La3+ ions are symmetrical, an octahedron coordinated by eight oxygen ions and isolated by two Ca−O6 octahedrons. When all of the La3+ ions are substituted with Eu3+ ions, Eu3+ ions are located at the center of the

⎡ 3V ⎤1/3 R c ≈ 2⎢ ⎥ ⎣ 4πxcN ⎦

(1)

Here, V is the volume of the unit cell, xc is the critical concentration, and N is the number of the host cations in the unit cell. For CEBO host, xc = 100%, N = 2, and VCEBO = 467.42 Å3. Thus, the calculated critical distance Rc between the Eu3+ ions in CEBO is about 7.64 Å which is larger than those in CaIn2O4:0.5%Eu3+ (3.22 Å) and Y2O3:0.5%Eu3+ (2.42 Å).3 Generally, exchange interaction or multipole−multipole interaction in oxide phosphors comes into effect only when the distance between activators is shorter than 5 Å in oxide phosphors.25 Therefore, excitation migration between Eu3+ ions in the 5D0 emitting state is neglectable, and no concentration quenching occurs even when 100% of La3+ ions are substituted with Eu3+ ions. 3.4. QY and Temperature-Dependent Photoluminescence Properties of CLBO:0.2%Eu and CEBO. The phosphors are placed in the integrated sphere, and excited with a monochromatic source of wavelength 340 nm. The absorbance and QY of the phosphors were measured and calculated by the method provided by HORIBA Jobin Yvon Inc.26 The phosphor absorbance, A, is

A=

L b − Lc Lb

(2)

Here, Lb is the integrated excitation profile when the sample is diffusely illuminated by the integrated sphere’s surface, and Lc is the integrated excitation profile when the sample is directly excited by the incident beam. The term La is the integrated excitation profile from an empty integrated sphere (without the sample). Eb is set equal to zero, and Lb then becomes the same as La because the secondary absorption and emission from the sample (that is, when the sample receives diffuse, and no direct, excitation light only) can be ignored using the parameters Lb = La = 150.3 and Lc = 65.6. As shown in Figure 4, the absorption of yellow phosphor CLBO:0.2%Eu3+ is 39.2%. The quantum yield was calculated by the following equation Φf =

Ec − (1 − A) ·E b La ·A

(3)

where Ec is the integrated luminescence of the sample caused by direct excitation, and Eb is the integrated luminescence of the sample caused by indirect illumination from the sphere. Using the experimental results, Ec = 1.43 × 106, Eb = 3.50 × 106, and La = 6.75 × 106, giving a quantum yield for CLBO:0.2%Eu3+ which is 46.2%. As shown in Figure 5a, CEBO possesses the highest intensity in the series of samples Ca4La1−xO(BO3)3:xEu3+ (with x = 0.03−1.0), and its luminescence intensity is higher than that of commercial red phosphor Y2O3:Eu3+ (provided by KeHeng Corp.). Similarly, the intensities of absorption of CEBO that originated from both CTB and 4f → 4f transitions are much higher than those of Y2O3:Eu3+ as observed in Figure S10. In Figures S11 and S12, by the above measurement method, the QYs of an as-prepared CEBO sample and Y2O3:Eu3+ are 98.1% and 83.4%, respectively. The temperature dependence of emission intensities of CEBO and Y2O3:Eu3+ have been comparatively investigated. The integrated intensities of E

DOI: 10.1021/acs.inorgchem.6b01786 Inorg. Chem. XXXX, XXX, XXX−XXX

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3.5. Long-Lasting Phosphorescence of CLBO:Eu and Its Mechanism. Long-lasting decay curves of Ca4La1−xO(BO3)3:xEu3+ (x = 0.005, 0.01, and 0.03) were acquired immediately after the corresponding samples were exposed to 331 nm irradiation dispersed from a xenon lamp as shown in

Figure 4. Excitation lines and emission spectrum of CLBO:0.2%Eu3+ measured using an integrating sphere. Note: The terms La and Lc are the integrated excitation profile from an empty integrated sphere (without the sample) and when the sample is directly excited by the incident beam, respectively. Eb and Ec are the integrated luminescence of the sample caused by indirect illumination from the sphere and by direct excitation. Figure 6. Afterglow decay spectra of phosphors Ca4La1−xO(BO3)3:xEu3+ (x = 0.005, 0.01, and 0.03) measured after the excitation source (the sample was exposed to 331 nm from Xe lamp for 1 min before measurement) is switched off.

Figure 6. All of the curves are well-fit with a biexponential formula as follows:27,28 ⎛ t ⎞ ⎛ t⎞ I(t ) = I0 + A1 exp⎜ − ⎟ + A 2 exp⎜ − ⎟ ⎝ τ2 ⎠ ⎝ τ1 ⎠

(4)

Here, I(t) represents the persistent luminescence intensity at time t. A1 and A2 are constants, and τ1 and τ2 are the decay times for the exponential components which could be associated with the rapid decay process first and then the slow one in the LLP materials, respectively. Apparently, τ2 chiefly determines the persistent performance of LLP materials. As shown in Figure 6, it could be seen that both τ1 and τ2 values gradually decrease with an increase of Eu3+ concentration. τ1 decreases from 12.78 to 8.44 ms, and τ2 decreases from 86.04 to 62.47 ms with the Eu3+ concentration increasing from 0.5 to 3.0 mol %, which is due to the increasing possibility of interactions between energy traps. Figure 7a exhibits emission spectra of the CLBO:0.2%Eu3+ sample measured under excitation at 331 nm and after being exposed to 331 nm excitation dispersed from the xenon lamp (then it is switched off), respectively. The features of both emission spectra in the persistent luminescence spectrum and steady emission spectrum (red line) are almost identical, which is more clearly shown in Figure S14, indicating that the LLP luminescence originates from the Eu3+ luminescence centers. The major LLP emission peaks are located at 537, 554, 585, and 610 (623) nm which are assigned to 5D1→ 7F1, 5D1 → 7F2, 5 D0 → 7F1, and 5D0 → 7F2 transitions of Eu3+ ions, respectively.1−5 It is generally believed that traps with suitable depth are essential for the LLP luminescence. Too shallow trapping levels could not efficiently stabilize the charges and lead to a fast decay and a short duration of emission. In contrast, the captured electrons could not easily escape at room temperature if the trapping levels are too deep. For better understanding the

Figure 5. (a) Emission spectra (excited at 395 nm) and (b) dependence of emission intensities of as-prepared sample Ca4EuO(BO3)3 and commercial red phosphor Y2O3:Eu3+ (provided by KeHeng Corp.) with measurement temperature.

CEBO measured at every temperature are higher than those of Y2O3:Eu 3+. As shown in Figure S13, the QY of the Ca4La1−xO(BO3)3:xEu3+ is increasing with the Eu3+ concentration increasing, which is consistent with the changing tendency of emission intensities as shown in Figure 3c. F

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Figure 7. (a) Thermoluminescent (TL) spectra of samples with long-lasting phosphorescence. (b) The three-dimensional TL spectra CLBO:0.2% Eu3+. (c) Emission spectra of CLBO:0.2%Eu3+ with and without excitation source. (d) The possible enhancement afterglow process of CLBO:0.2% Eu3+.

half width, respectively. The asymmetry parameter μg = T1/ω. The traps at 56, 78, and 113 °C are 0.72, 0.58, and 0.3 eV, respectively.30,31 The three-dimensional TL spectra CLBO:0.2%Eu3+ in Figure 7c show that the LLP luminescence characteristics mostly originated from the transitions 5D1,2−7FJ of Eu3+ with a maximum wavelength at 585 nm. On the basis of the above results and discussion, the possible mechanism of LLP could be put forward and schematically illustrated in Figure 7d. Under UV excitation, the electrons located at the ground state of Eu3+ (7FJ) would be promoted to the conduction band (CB) (process 1), while the holes leave in the valence band (VB). The excited electrons could freely move in CB, and part of them transfer to the excited level 5D0 of Eu3+ (process 2). Finally, the excited electrons would move to the ground state, and the luminescence emission appears as follows (process 3). The other activated electrons in CB would be captured and stabilized by the energy traps. When the external irradiation ceases, the electrons captured by the traps would be released and return to the excited state with the aid of lattice vibrations caused by the suitable region of temperature agitation. Subsequently, the recombination of electrons and holes produces LLP luminescence (process 4). The afterglow decay spectra of orange phosphor CLBO:0.2% Eu3+ (monitored at 537, 555, 585, 615, and 623 nm) measured after the excitation source being switched off are shown in Figure S15. All of the decay curves can be well-fit into a biexponential function. It is observed that the decay time of emissions due to transitions from the higher energy levels 5D1,2 is shorter than those from low energy level 5D0. The electron at higher energy levels is metastable and would readily return to the ground states 7FJ or relax to the low energy level 5D0 by multiphonon nonradiation.2,3

trap properties in the CLBO:0.2%Eu3+ and CLBO:0.2% Eu3+,RE3+ (RE = Dy, Er) samples, the TL measurement was carried out from 20 to 200 °C. As presented in Figure 7b, one asymmetric and broad TL band with a center at about 66 °C could be observed in all three samples: CLBO:0.2%Eu3+ and CLBO:0.2%Eu3+, RE3+ (RE = Dy, Er), which is possibly due to the intrinsic defects caused by the slight difference of ionic radii between Eu3+ and La3+. No obvious shift of the TL peak center could be detectable, but the TL intensity is improved by codoping Dy3+ and Er3+. The results indicate that Dy3+ or Er3+ codoped into the lattice host did not create new energy trapping levels, but more electrons are captured to bring stronger persistent luminescence. The TL glow curve of the CLBO:0.2%Eu3+ sample could be well-deconvoluted into three separated components based on Gaussian function with R2 = 0.99908 (R is a coefficient of determination, and R2 measures goodness of fit). The segregated Gaussian profiles are, respectively, centered at about 57, 78, and 111 °C which are all situated between 320 and 400 K, an appropriate temperature region for a highly persistent performance for LLP materials. An intense LLP luminescence could be observed in the CLBO:0.2%Eu3+ sample by the naked eye at room temperature. The three TL bands suggest that there exist three types of traps with different depths in CLBO:0.2%Eu3+.28,29 The depths of the traps are calculated by following function: ⎛ K T2 ⎞ E = [2.52 + 10.2(μg − 0.42)]⎜ B m ⎟ − 2KBTm ⎝ ω ⎠

(5)

Here, E is the trap depth; Tm is the temperature corresponding to the TL glow peak. kB is the Boltzmann constant (1.38 × 10−23 J K−1), and ω is the shape parameter and is defined as ω = T2 − T1, with T1 and T2 being the high and low temperature G

DOI: 10.1021/acs.inorgchem.6b01786 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

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4. CONCLUSION In conclusion, a Eu3+ doped phosphor CLBO:Eu has been synthesized by a combustion method using urea and followed by sintering at 900 °C. The highest luminescence intensity of CLBO:Eu is achieved when all of the La3+ ions is substituted with Eu3+. The absence of concentration quenching due to a particularly large distance in CLBO structure even when the doping concentration of Eu3+ is 100 mol % of La3+. Emissions from the 5D1 level are strong from the phosphor CLBO:0.2% Eu. It is worth noticing that the nearest neighboring Eu−Eu distance is long enough to prevent concentration quenching in 5 D0, but still interact with each other to enable cross-relaxation from 5D1. The red phosphor Ca4EuO(BO3)3 has a quantum yield as high as 98% that is much higher than that of the commercial green phosphor Y2O3:Eu3+. The orange phosphor CLBO:0.2%Eu has a quantum yield as high as 46.2%. These results indicate that the large critical Eu−Eu distance in CLBO structure is vital for reducing exchange interaction of Eu3+ from higher energy levels 5D1,2. The red phosphor CLBO:Eu emits LLP luminescence lasting for about 2 h after the excitation source was turned off. Codoping Dy3+ and/or Er3+ ions prolonged the duration of the LLP luminescence of the afterglow in the CLBO:Eu system is presented.

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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.6b01786. Rietveld XRD pattern, SEM, EDS, excitation (λex = 266 nm) and emission (λem = 610 and 623 nm) spectra of CLBO:0.2%Eu, PL spectra, ratio 5D0 → 7F1/5D0 → 7F2, decay times and QY data, comparison of spectra and QY, afterglow emission spectra and decay (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Fax: +86-577-8837-3017. Phone: +86-577-8837-3017. *E-mail: [email protected]. Author Contributions

All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was jointly supported by the National Natural Science Foundation of China (51572200, 51102185) and Zhejiang Province (Y16E020041), and the Public Industrial Technology Research Projects of Zhejiang Province (2015C33142) and Wenzhou City (G20140040).

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REFERENCES

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DOI: 10.1021/acs.inorgchem.6b01786 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

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DOI: 10.1021/acs.inorgchem.6b01786 Inorg. Chem. XXXX, XXX, XXX−XXX