Changing Ce3+ Content and Codoping Mn2+ Induced Tunable

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Changing Ce3+ Content and Codoping Mn2+ Induced Tunable Emission and Energy Transfer in Ca2.5Sr0.5Al2O6:Ce3+,Mn2+ Mengqiao Li,†,‡ Jilin Zhang,*,†,‡ Jin Han,†,‡ Zhongxian Qiu,†,‡ Wenli Zhou,†,‡ Liping Yu,†,‡ Zhiqiang Li,†,‡ and Shixun Lian*,†,‡ †

Key Laboratory of Chemical Biology and Traditional Chinese Medicine Research (Ministry of Education of China) and ‡Key Laboratory of Sustainable Resources Processing and Advanced Materials of Hunan Province College, Hunan Normal University, Changsha 410081, China S Supporting Information *

ABSTRACT: A series of color-tunable Ce3+ single-doped and Ce3+, Mn2+ codoped Ca2.5Sr0.5Al2O6 phosphors were synthesized by a high-temperature solid-state reaction. The crystal structure, luminescent properties, and energy transfer were studied. For Ca2.5Sr0.5Al2O6:Ce3+ phosphors obtained with Al(OH)3 as the raw material, three emission profiles were observed. The peak of photoluminescence (PL) spectra excited at ∼360 nm shifts from 470 to 420 nm, while that of the PL spectra excited at 305 nm stays unchanged at 470 nm with the increase of Ce3+ content. Furthermore, the peak of PL spectra is situated at 500 nm under excitation at ∼400 nm. The relationship between the luminescent properties and crystal structure was studied in detail. Ce3+, Mn2+ codoped Ca2.5Sr0.5Al2O6 phosphors also showed interesting luminescent properties when focused on the PL spectra excited at 365 nm. Obvious different decreasing trends of blue and cyan emission components were observed in Ca2.5Sr0.5Al2O6:0.11Ce3+,xMn2+ phosphors with the increase in Mn2+ content, suggesting different energy transfer efficiencies from blue- and cyan-emitting Ce3+ to Mn2+. Phosphors with high color-rendering index (CRI) values are realized by adjusting the doping content of both Ce3+ and Mn2+. Studies suggest that the Ca2.5Sr0.5Al2O6:Ce3+,Mn2+ phosphor is a promising candidate for near UV-excited w-LEDs.

1. INTRODUCTION White-light-emitting diodes (w-LEDs) are the new generation of lighting and have advantages over traditional incandescent and fluorescent lamps, such as high luminous efficiency, energy savings, and environmental friendliness.1 Currently, common w-LEDs are made of phosphor-converted LEDs using a blue LED in combination with a yellow-emitting Y3Al5O12:Ce3+ phosphor.2,3 However, this type of w-LED has weaknesses, such as a poor color-rendering index (CRI) due to the lack of red-emitting component.4 The combination of a blue LED with green and red phosphors or a near-UV LED with blue, green, and red tricolor phosphors can generate a high CRI value. However, such combinations still show a disadvantage wherein the degradation rate of one phosphor is different from the other. A full-color single-phase white phosphor could overcome the problems mentioned above.5,6 Therefore, there is great demand for near-UV light excitable single-phase white phosphors with a high CRI value. Shang et al. summarized four main methods to obtain white light in a single-phase host, viz., (1) doping a single rare earth (RE) ion (Eu2+/Eu3+/Dy3+) in an appropriate host, (2) doping multiple RE ions that are excited simultaneously, (3) codoping different ions (Ce3+− Mn2+ codoping, Eu2+−Mn2+ codoping, etc.) with energy transfer, and (4) defect-related luminescent materials.7 Among such methods, the method based on energy transfer © XXXX American Chemical Society

not only can control the emission colors more easily but also can achieve higher luminous efficiency. Generally, Ce3+ and Eu2+ ions are used as sensitizers that can transfer their excited energy to an activator such as Mn2+.8−16 This is because the 4f−5d transitions in Ce3+ and Eu2+ ions are allowed and result in strong excitation/absorption intensity. Furthermore, the excitation and emission bands of Ce3+ and Eu2+ ions can be tuned by changing their coordination environment in the same host or selecting another host. There are mainly three methods to change the coordination environment of activators in a host. The first one is achieved by partial substitution of the cation that will be occupied by the activators, for example, an isostructural solid solution of (CaMg) x (NaSc) 1−x Si 2 O 6 , 17 (Sr 1−x ,Ba x ) 3 MgSi 2 O 8 :Eu 2+ , 18 Ba3−xCaxSi6O12N2:Eu2+,19 (Ba,Sr)3Lu(PO4)3:Eu2+,20 etc. The second one is achieved by partial substitution of the anion group, for example, Na3K(Si1−xAlx)8O16±δ:Eu2+,21 a double substitution of Mg2+−Si4+/Ge4+ for Al3+−Al3+ in Ce3+-doped garnet phosphor,22 nitridation of the host,23 partial substitution of Si4+−O2− for Al3+−F−, or vice versa.24 The third one is achieved by substitution of another crystallographic site in the host that has more than one site suitable for activators, for Received: August 27, 2016

A

DOI: 10.1021/acs.inorgchem.6b02082 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry example, doping content induced tailoring of the luminescence in α′L-Ca2SiO4:Eu2+.25 M3Al2O6 (M = Ca, Sr) has attracted much attention as a host for Ce3+/Eu2+-doped phosphors in the past years. For example, blue (460 nm)26 and yellow-green (536 nm)27 emitting Sr3Al2O6:Ce3+, green (514 nm)28 and red (604 nm)28 emitting Sr3Al2O6:Eu2+, blue (418 nm), cyan (470 nm), and yellow (550 nm) emitting Ca3Al2O6:Ce3+,29 and deep red emitting Ca3Al2O6:Eu2+30 under different excitation wavelengths are reported, which is due to several M sites being suitable for the luminescent centers. Single-phase white-emitting Ca 2 SrAl 2 O 6:Ce 3+,Li + ,Mn 2+ and CaSr 2Al 2 O 6 :Ce 3+,Li+ ,Mn 2+ phosphors via energy transfer are also reported.31,32 In these two white-emitting phosphors, the main excitation band is situated at ∼360 nm when monitoring the main emission band at 470 nm. However, our previous research suggested that the main excitation band is at ∼305 nm for the emission band peaking at 470 nm, while a blue emission band was observed under excitation at ∼360 nm.29,33 Therefore, the relationship between Ce3+-related luminescent properties and crystal structure of M3Al2O6 deserves further investigation. In the present work, a solid solution of Ca2.5Sr0.5Al2O6 is selected as the host for Ce3+ single-doped and Ce3+, Mn2+ codoped phosphors. The effect of introducing Sr2+ and Ce3+ content on the photoluminescent excitation and emission spectra (PLE and PL spectra) of a Ce3+-doped phosphor is discussed. Site-sensitive energy transfer from Ce3+ to Mn2+ is observed in Ca2.5Sr0.5Al2O6. A proper method for single-phase white-emitting Ca2.5Sr0.5Al2O6:Ce3+,Li+,Mn2+ with a high CRI value is also put forward.

2. EXPERIMENTAL SECTION 2.1. Materials and Synthesis. Phosphors with the nominal compositions of Ca2.5−2xSr0.5Al2O6:xCe3+,xLi+ (CSAO:xCe3+, x = 0.01−0.24) and Ca2.5−2a−xSr0.5Al2O6:aCe3+,aLi+,xMn2+ (CSAO:aCe3+,xMn2+, a = 0.05, 0.10, and 0.11, x = 0.01−0.07) were prepared by a high-temperature solid-state reaction. The starting materials CaCO3 (A.R.), SrCO3 (A.R.), Al(OH)3 (A.R.), CeO2 (99.99%), Li2CO3 (A.R.), and MnCO3 (A.R.) with stoichiometric amounts were mixed thoroughly with alcohol and ground for 30 min in an agate mortar. The mixed powders were placed in an alumina crucible and calcined at 1350 °C for 4 h under a reductive atmosphere (5% H2 + 95% N2) and then cooled to room temperature naturally. 2.2. Measurements and Characterization. The powder X-ray diffraction (XRD) data were collected on a PANalytical X’Pert Pro diffractometer with Cu Kα radiation (λ = 1.540 56 Å) operated at 40 kV and 40 mA. The Rietveld structure refinements were performed by using the General Structure Analysis System (GSAS) software.34 The morphology of phosphors was observed by using a JEOL JSM-6490LV scanning electron microscope (SEM). The photoluminescence (PL) and photoluminescence excitation (PLE) spectra of CSAO:xCe3+ phosphors were collected on a Hitachi F-4500 spectrophotometer. The PL spectra of CSAO:aCe3+,xMn2+ and fluorescence lifetime were measured on an Edinburgh FLS920 combined fluorescence lifetime and steady-state spectrometer. Temperature-dependent PL spectra were collected on a Hitachi F-4500 spectrophotometer equipped with a high-temperature controller. All the measurements were performed at room temperature except the temperature-dependent PL spectra.

Figure 1. XRD patterns of selected phosphors. (a) CSAO:xCe3+, (b) CSAO:0.05Ce3+,xMn2+, and (c) CSAO:0.11Ce3+,xMn2+.

CSAO:xCe3+ samples are all of a pure phase, whose XRD patterns are consistent with the standard card of Ca3Al2O6 (JCPDS 38-1429). The introduction of Sr2+ and Ce3+ does not change the phase. The magnified diffraction peak at around 33° shows that the diffraction peak of Ca2.5Sr0.5Al2O6 is smaller than that of Ca3Al2O6. Furthermore, the diffraction peak shifts to the higher angle side with the increase of Ce3+ content. These results suggest that the introduction of Sr2+ with a larger radius results in the increase of interplanar distances, while the doping of Ce3+ leads to the decrease of these distances. The XRD patterns of CSAO:0.05Ce3+,xMn2+ and CSAO:0.11Ce3+,xMn2+ are shown in Figure 1b and c, respectively, which also exhibit a single-phase feature. The SEM images of CSAO:0.05Ce3+ and CSAO:0.11Ce3+ are shown in Figure S1. The morphology suggests that both samples have irregular particles, which is the characteristic morphology of phosphors obtained by a traditional hightemperature solid-state reaction. The sizes of both samples are in micrometer scale, and particles are agglomerated. Furthermore, the degree of agglomeration for CSAO:0.11Ce3+

3. RESULTS AND DISCUSSION 3.1. Phase, Morphology Characterization, and Structure Refinement. Phase purity of CSAO:xCe 3+ and CSAO:Ce3+,xMn2+ phosphors was identified by collecting XRD patterns. The XRD patterns of representative samples are shown in Figure 1. As shown in Figure 1a, the selected B

DOI: 10.1021/acs.inorgchem.6b02082 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry seems higher than that of CSAO:0.05Ce3+. CSAO:0.11Ce3+ used a greater amount of Li2CO3 to balance the charge. On the other hand, Li2CO3 could also act as a flux. Therefore, the amount of Li2CO3 may be the main reason for the different particle sizes. XRD Rietveld refinement of CSAO:0.05Ce 3+ and CSAO:0.11Ce3+ was performed on the basis of the crystallographic data of Ca3Al2O6 (ICSD 1841) by using the GSAS program to get more information on the crystal structure. The reported crystal structure of Ca3Al2O6 suggests that there are six different Ca2+ sites in a unit cell.35 The coordination numbers of oxygen for each site are 6 [Ca(1)], 6 [Ca(2)], 6 [Ca(3)], 9 [Ca(4), MO9], 8 [Ca(5), MO8], and 7 [Ca(6), MO7], and the numbers of each site in a unit cell are 4, 4, 8, 8, 24, and 24 for Ca(1), Ca(2), Ca(3), Ca(4), Ca(5), and Ca(6), respectively. The chemical formula Ca2.5Sr0.5Al2O6 suggests that one-sixth of Ca atoms, namely, 12 Ca atoms, in a unit cell are substituted by Sr atoms. The Sr atoms tend to occupy the largest MO9 site first due to a larger ion radius. However, the number of atoms on the MO9 site is only 8 in a unit cell. Therefore, four Sr atoms left in a cell are going to occupy the second biggest site, namely, the MO8 site. Figure 2 illustrates

Table 1. Crystallographic Data, Refinement Parameters, and Selected Bond Lengths of CSAO:0.05Ce3+ and CSAO:0.11Ce3+ cryst syst space group a/Å V/Å3 Z Rwp/% Rp/% χ2

CSAO:0.05Ce3+

CSAO:0.11Ce3+

cubic Pa3̅ 15.3470(3) 3614.67(6) 24 5.02 3.78 1.801

cubic Pa3̅ 15.3478(4) 3615.25(9) 24 5.84 4.12 2.428

Supporting Information. Refinement results suggest that more than four-fifths of the MO9 site and one-fifth of the MO8 site are occupied by Sr atoms, while Ce3+ ions on MO9 and MO8 sites have nearly equal occupancy. The amount of Ce3+ ions on both MO9 and MO8 sites increases with Ce3+ content. It should be noticed that the number of MO8 sites is 3 times that of MO9 sites. A unit cell based on the refinement result of CSAO:0.05Ce3+ is illustrated in Figure 3, which also shows the AlO4 tetrahedra and coordination environment of MO9 and MO8 sites.

Figure 3. Crystal structure of CSAO, showing (a) the AlO4 tetrahedra and (b) the coordination environment of MO9 and MO8 sites.

3.2. Photoluminescence Properties. 3.2.1. Photoluminescence Properties of CSAO:xCe3+ Phosphors. The emission and excitation spectra of CSAO:xCe3+ phosphors are illustrated in Figure 4 and Figure 5. The PL spectra of selected phosphors under 360 nm excitation are shown in Figure 4a. For low doping content of Ce3+, the emission band is located at about 470 nm with a shoulder band at around 500 nm and a weaker, overlapped band at around 420 nm. The main peak of the emission band shifts to shorter wavelength with the increase of Ce3+ (for example 0.11Ce3+). It is clear that the emission band at 470 nm dominates for low doping content, while the emission band at around 420 nm dominates for higher doping

Figure 2. Rietveld refinement of the powder XRD patterns of (a) CSAO:0.05Ce3+ and (b) CSAO:0.11Ce3+.

the experimental and refinement results of XRD profiles for CSAO:0.05Ce3+ and CSAO:0.11Ce3+. Crystallographic data and refinement parameters are collected in Table 1. The unit cell volumes of both CSAO:0.05Ce3+ and CSAO:0.11Ce3+ are larger than that of pure Ca3Al2O6 (3555.66 Å3). The crystal data can be found in the CIF files and Tables S1 and S2 in the C

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Figure 4. PL spectra of CSAO:Ce3+ phosphors: (a) original, (b) normalized PL spectra excited at 360 nm, and (c) related PL intensity at 420 and 500 nm versus Ce3+ content; (d) original, (e) normalized PL spectra excited at 305 nm, and (f) corresponding PL intensity at 470 nm versus Ce3+ content.

Figure 5. PLE spectra monitored at (a) 470 nm and (b) 420 nm, (c) PL spectra excited at 400 nm, and (d) PLE spectra monitored at 500 nm in CSAO:Ce3+ phosphors.

content. The evaluation of emission peak and intensities at both 420 and 500 nm is illustrated in Figure 4b and c. The PL spectra of selected phosphors with different Ce3+ contents under 305 nm excitation are shown in Figure 4d, which have a similar emission band peaking at 470 nm. Normalized PL spectra and the intensity versus Ce3+ content are shown in Figure 4e and f, respectively. The full-width at half-maximum (fwhm) of the emission band at 470 nm tends to decrease with the increase in Ce3+ content. The PLE spectra monitored at 470 and 420 nm are shown in Figure 5a and b, respectively. It is obvious that the main excitation band of both PLE spectra is situated at around 360 nm, and they both contain a shoulder PLE band at about 300− 330 nm. However, there are two obvious differences between

these two PLE spectra. First, the ratio of PLE intensity at 300 to 360 nm for the spectra monitored at 470 nm is larger than that monitored at 420 nm. Second, the PLE spectra for phosphors with higher doping content monitored at 470 nm have an additional PLE band at about 400 nm, which reaches a maximum when the content of Ce3+ is 0.08 and even nearly disappears with x higher than 0.18, while the PLE band at 400 nm is not so distinct when monitored at 420 nm. The PL spectra excited at 400 nm are illustrated in Figure 5c with an emission band peaking at ∼500 nm, whose profiles are different from that excited at 305 nm. However, the PLE spectra monitored at 500 nm as shown in Figure 5d are similar to those monitored at 470 nm. This is because the emission bands D

DOI: 10.1021/acs.inorgchem.6b02082 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 6. Normalized PL spectra and corresponding Gaussian fitting results of CSAO:0.05Ce3+ (left column) and CSAO:0.11Ce3+ (right column). (a, b) Normalized PL spectra and fitting results of PL spectra excited at (c, d) 305 nm, (e, f) 400 nm, and (g, h) 360 nm.

be lower than that of newly formed Al2O3 from the decomposition of Al(OH)3. The moving of Ce3+ ions is slow when neutral Al2O3 is used. Therefore, Ce3+ ions mainly occupy the CaO7 site that has the largest number in a unit of Ca3Al2O6, because there is not enough time for Ce3+ ions to move to a more stable site, namely, CaO9 and CaO8. Ce3+ ions in Ca2.5Sr0.5Al2O6 should have a similar result due to a similar crystal structure. A CSAO:0.03Ce3+ phosphor is prepared by utilizing neutral Al2O3 as raw material in a similar solid-state reaction, whose PL and PLE spectra are illustrated in Figure S2, indicating the achievement of yellow emission peaking at 550 nm under 470 nm excitation. Therefore, the observed emission band at 500 nm with a corresponding excitation band at 400 nm should originate from Ce3+ with an environment other than normal MO9, MO8, and MO7 sites. The crystallographic sites left for Ce3+ ions are CaO6 sites. However, it seems impossible, because Ce3+ ions on such small sites relate to an even longer emission peak according to the equation. It could be noticed that the emission band at 500 nm is quite close to that at 470 nm. The crystallographic sites of these two kinds of Ce3+ ions thereby have some correlation. When Sr2+ ions enter into the MO8 site, the coordination polyhedra containing CaO8 may be compressed due to a much larger ion radius of Sr2+, which can be called a remote control effect.37 Ce3+ ions on a contractive CaO8 site may have a longer emission wavelength according to eq 1. Furthermore, the contractive CaO8 may lead to a different splitting of 5d levels, and therefore an additional PLE band at ∼400 nm is reasonable. Ce3+ tends to occupy the normal CaO8 site when the Ce3+ content is low, which results in a weak emission under excitation at 400 nm. The amount of Ce3+ on a contractive CaO8 site increases with doping content, and the emission intensity increases until concentration quenching. The phenomenon can also be explained as follows. The introduction of Sr2+ to a MO8 site will result in two different MO8 polyhedra with different sizes. One is related to Sr2+, viz., SrO8, and the other is CaO8. Therefore, Ce3+ ions on MO8 should have two different emission bands. However, the difference between SrO8 and CaO8 cannot be obtained through the structural refinement. Normalized PL spectra and corresponding Gaussian fitting results of the three PL profiles for both CSAO:0.05Ce3+ and CSAO:0.11Ce3+ phosphors are shown in Figure 6. Normalized PL spectra in Figure 6a and b show clear, different profiles within a phosphor under different excitation wavelengths and

obtained under excitation at 305, 360, and 400 nm overlap heavily around 470−500 nm. The PL spectra mentioned above indicated that there are three different kinds of emission bands in CSAO:xCe3+ phosphors with maximum intensity at around 420, 470, and 500 nm, respectively. Especially the emission band excited at 360 nm shifts from ∼470 nm to ∼420 nm with increasing the content of Ce3+. It is well-known that the emission band of Ce3+ originating from the 5d−4f transition is sensitive to the coordination environment. Therefore, Ce3+ ions should situate at three different coordination environments. When excited at around 360 nm, CSAO:0.05Ce3+ and CSAO:0.11Ce3+ exhibit the highest PL intensity at 470 and 420 nm, respectively, among all the Ce3+-doped CSAO phosphors. These two phosphors are selected for the following studies, consequently, to gain a deep understanding of the relationship between evolution of luminescent property and crystal structure. Our previous work observed three emission bands peaking at ∼418, 470, and 547 nm in the Ca3Al2O6 host, which were assigned to Ce3+ ions on CaO9, CaO8, and CaO7 sites,29 respectively, based on the calculation of the energy difference (E) between the lowest excited 5d state and 4f ground state using the following equation:36 ⎡ ⎤ ⎛ V ⎞1/ V E = Q ⎢1 − ⎜ ⎟ × 10−nEar /80⎥ ⎝4⎠ ⎢⎣ ⎥⎦

(1)

where Q is the lower d-band edge for free Ce3+ ion (50000 cm−1), V and n are the valence and coordination number of Ce3+, respectively, Ea is the electron affinity of O that forms O2−, and r is the radius of Ca2+ or Sr2+ in CSAO replaced by Ce3+. The value of Ea for O is 1.60 in the host CSAO according to ref 36. The values of r in MO9 and MO8 sites are obtained from the average bond length of M−O minus the radius of O2−. Structural refinement results of CSAO:0.05Ce 3+ and CSAO:0.11Ce3+ suggest that Ce3+ ions are on MO9 and MO8 sites. Due to the similarity in crystal structure of Ca3Al2O6 and Ca2.5Sr0.5Al2O6, the observed emission bands at around 420 and 470 nm in the present work can be assigned to Ce3+ on MO9 and MO8 sites, respectively. The enhancement in the 420 nm band with the increase of Ce3+ is consistent with the increase of Ce3+ on the MO9 site. Our previous work showed that Ca3Al2O6:Ce3+ exhibited a strong yellow emission at ∼550 nm excited by a blue light if neutral Al2O3 was used as the raw material.29 The reactivity of neutral Al2O3 raw material should E

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viewpoint as shown in Figure 7. When studying the lifetime of the excited state of the blue-emitting part, 400 nm is selected as

different profiles between the two phosphors under excitation at 360 nm. The PL band excited at 305 nm has the simplest and intact profile, so its corresponding Gaussian fitting results are obtained first. The Gaussian fitting results of the PL band excited at 360 nm are obtained last. Detailed fitting results are described as follows. (1) The PL band excited at 305 nm for both phosphors can be fitted to three Gaussian bands at around 400, 470, and 514 nm as shown in Figure 6c and d. The energy differences between the two Gaussian bands around 470 and 514 nm are 1636 and 1825 cm−1 for CSAO:0.05Ce3+ and CSAO:0.11Ce3+, respectively. These values are acceptable because the energy difference between the 2 F5/2 and 2F7/2 sublevels of the 4f1 ground state in Ce3+ is around 2000 cm−1 due to spin−orbit coupling.38,39 The energy difference between the bands at around 400 and 470 nm is as high as 3000−3500 cm−1, which suggests that the Gaussian bands at 400 and 470 nm belong to Ce3+ ions with different coordination environments. (2) The PL band excited at 400 nm for both phosphors can be fitted to two bands at around 489 and 534 nm, which are shown in Figure 6e and f. The energy differences between the two Gaussian bands are 1748 and 1719 cm−1 for CSAO:0.05Ce3+ and CSAO:0.11Ce3+, respectively. (3) The Gaussian fitting result of the PL profile excited at 360 nm for CSAO:0.05Ce3+ is shown in Figure 6g, which exhibits four Gaussian bands at ∼420, 470, 516, and 566 nm. The bands at 470 and 516 nm are similar to those under excitation at 305 nm, which suggests that they originate from a Ce3+ ion on the same crystallographic site, namely, the normal CaO8 site. The band at 420 nm can be assigned to a Ce3+ ion that is on the MO9 site and should be fitted to two Gaussian bands due to the spin− obit splitting of 4f1 for Ce3+. However, it is hard to get such a fitting result because of the weak emission compared with the main emission band. The fitted band at 566 nm may originate from Ce3+ on a contractive CaO8 site that can exhibit a PL band peaking at around 500 nm when excited at 400 nm. Because its emission intensity is low when excited at 360 nm and its PL band overlaps the band around 470 nm, it is also hard to get two Gaussian bands. (4) The Gaussian fitting result of the PL profile excited at 360 nm for CSAO:0.11Ce3+ as shown in Figure 6h contains five bands situated at ∼397, 426, 472, 517, and 569 nm. Undoubtedly the resulting bands at 472 and 517 nm originate from Ce3+ on a normal CaO8 site. The bands at 397 and 426 nm have an energy difference of 1723 cm−1. Therefore, these two bands belong to Ce3+ on a MO9 site. The band at 569 nm is assigned to Ce3+ on a contractive CaO8 site. Our previous papers demonstrated that the PLE spectrum monitored at 470 nm in Ca3Al2O6:Ce3+ contained only two overlapped bands, at 305 and 335 nm, and had no band at ∼360 or 400 nm, while the PLE spectrum monitored at 418 nm showed a strong excitation band at ∼360 nm.29,33 In the present CSAO:Ce3+ phosphor, the PLE spectra monitored at 420 and 470 nm both exhibit a maximum excitation at ∼360 nm. Therefore, it is expected that there is energy transfer from blue-emitting Ce3+ on the MO9 site to cyan-emitting Ce3+ on the MO8 site. Decay curves are measured to support this

Figure 7. Fluorescence decay curves of selected CSAO:Ce 3+ phosphors monitored at (a) 400 nm and (b) 470 nm.

the monitoring wavelength to reduce the interference from the cyan-emitting Ce3+. Figure 7a illustrates the decay curves of CSAO:0.11Ce3+ and CSAO:0.18Ce3+ monitored at 400 nm. The curves can be well fitted with a double-exponential function:40 I = A1 exp( −t /τ1) + A 2 exp(−t /τ2)

(2)

The average lifetime (τ) is obtained using the following function t = (A1τ12 + A 2 τ2 2)/(A1τ1 + A 2 τ2)

(3)

The average lifetimes for these two phosphors are given in Figure 7a. The resulting double-exponential function may suggest that there are other ways for the excited electrons to return to the ground state besides the radiative and nonradiative ways. Energy transfer is expected from blue-emitting Ce3+ to a cyan one according to the PLE spectra; however, concentration quenching cannot be excluded. Figure 7b illustrates the decay curves of CSAO:Ce3+ monitored at 470 nm, which are all well fitted with a doubleexponential function. However, the decay curves monitored at 470 nm for Ca3Al2O6:Ce3+ in our previous work can all be fitted with a single-exponential function.29 Table 2 lists the fitting results of the decay curves based on eqs 2 and 3. It is obvious from the decay curves that the excited state decays Table 2. Fitting Results of Decay Curves for Selected CSAO:xCe3+ Phosphors Monitored at 470 nm

F

x

τ1/ns

τ2/ns

A1

A2

τ/ns

A1/A2

0.02 0.05 0.08 0.11 0.18 0.24

11.05 8.68 8.29 8.33 8.27 8.32

66.04 63.31 60.56 59.28 58.95 58.37

264 628.7 503 924.4 687 350.3 861 755.7 1 180 900 1 368 970

106 041.6 105 069.9 97 014.6 88 792.2 74 271.3 64 487.5

49.8 41.6 34.8 29.9 24.0 20.8

2.496 4.796 7.085 9.705 15.90 21.23

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Inorganic Chemistry faster with the increase of Ce3+ content, which is also indicated by the average lifetime (τ). However, the fitted lifetimes, viz., τ1 and τ2, change little with Ce3+ content. The shape of the decay curves strongly depends on the values of constants A1 and A2. Table 2 also lists the ratio of A1 to A2, which increases with Ce3+ content. The lower ratio suggests that A2-related decay (τ2) dominates the total decay curve, while the higher ratio suggests that A1-related decay (τ1) dominates the total decay curve. These phenomena are in good agreement with the evolution of PL spectra with Ce3+ content excited at 360 nm; namely, the 470 nm band dominates at lower Ce3+ content and the 420 nm band dominates at higher Ce3+ content. The PLE and PL spectra of CSAO:0.05Mn2+ are shown in Figure 8a. The PL spectrum is obtained under excitation at 420

nm, which has a broad emission band peaking at 610 nm. The PLE spectrum is obtained by monitoring at 610 nm, which contains several broad and sharp bands. The transitions can be designated as follows based on the Tanabe−Sugano diagram of Mn2+: 6A1 → 4T1 (4G) (550 nm), 6A1 → 4T2 (4G) (460 nm), 6 A1 → 4A1/4E (4G) (420 nm), 6A1 → 4T2 (4D) (370 nm), 6A1 → 4E (4D) (355 nm), 6A1 → 4T1 (4P) (320 nm). However, the emission intensity of CSAO:0.05Mn2+ is very low due to the forbidden 3d−3d transitions. In order to enhance this emission, energy transfer is often used based on codoping, for example, Ce3+ and Mn2+. The PL spectra of selected CSAO:Ce3+ phosphors are also illustrated in Figure 8b−d. It is obvious that all kinds of emission bands of Ce3+ overlap several PLE bands of Mn2+, which indicates that strong energy transfer can be expected from Ce3+ to Mn2+ in a CSAO host. 3.2.2. Photoluminescence Properties of CSAO:0.05Ce3+,xMn2+ and CSAO:0.11Ce3+,xMn2+ Phosphors. A series of PL spectra of CSAO:0.05Ce3+,xMn2+ phosphors are illustrated in Figure 9. Whether excited at 365 or 300 nm, the PL spectra of CSAO:0.05Ce3+,xMn2+ phosphors contain both a broad band at ∼470 nm and a broad band at around 630 nm (Figure 9a), which originate from Ce3+ and Mn2+, respectively. The emission intensity of Ce3+ decreases with the increase of Mn2+ content, while that of Mn2+ increases first, reaching a maximum when the content of Mn2+ is 0.05, and then decreases. Furthermore, the peak value of Mn2+ emission shifts from 628 nm to 638 nm with increasing the content of Mn2+ (under 365 nm excitation). The PLE spectra obtained by monitoring the emission of Mn2+ are shown in Figure 9c, which contain both the excitation band of Ce3+ at ∼365 nm and that of Mn2+ at ∼560 nm obviously. Therefore, the excitation band in the range of 300−400 nm should also contain those from the transition between energy levels in Mn2+ as discuss before. These results suggest that the red emission in CSAO:0.05Ce3+,xMn2+ phosphors originates from both energy transfer from Ce3+ to Mn2+ and self-excitation in Mn2+. The decay curves of CSAO:0.05Ce3+,xMn2+ phosphors monitored at 470 nm are shown in Figure 9d. It is obvious that the excited state of Ce3+ decays faster with an increase in Mn2+ content. The

Figure 8. (a) PLE and PL spectra of CSAO:0.05Mn2+ and (b−d) three representative PL spectra of selected CSAO:Ce3+ for analysis of spectral overlap.

Figure 9. PL spectra of CSAO:0.05Ce3+,xMn2+ phosphors excited at (a) 365 nm and (b) 300 nm, (c) PLE spectrum of CSAO:0.05Ce3+,0.02Mn2+, and (d) fluorescence decay curves of selected CSAO:0.05Ce3+,xMn2+ phosphors monitored at 470 nm. G

DOI: 10.1021/acs.inorgchem.6b02082 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 10. (a) PL spectra of CSAO:0.11Ce3+,xMn2+ phosphors excited at 365 nm, (b) PLE spectrum of selected CSAO:0.11Ce3+,xMn2+, and (c and d) fluorescence decay curves of selected CSAO:0.11Ce3+,xMn2+ phosphors monitored at 470 and 400 nm, respectively.

decay curves can be fitted well by a double-exponential function. The fitting results of the decay curves are listed in Table S3. The fitted lifetimes τ1 and τ2 both decrease with the increase of Mn2+ content, which is different from those in Ce3+ single-doped ones. The average lifetimes calculated by using eq 3 for selected phosphors are also listed in Figure 9d. The decay results suggest that there is an additional way for the excited state of Ce3+ to return to the ground state, indicating again the existence of energy transfer between cyan-emitting Ce3+ and Mn2+. The energy transfer efficiency (ηT) can be evaluated by using the following equation:41 ηT = 1 −

τx τ0

shown in Figure 10b. The profile suggests that energy transfer from Ce3+ to Mn2+ is the main reason for the strong red emission. Figure 10c and d illustrate the decay curves of CSAO:0.11Ce3+,xMn2+ phosphors monitored at 470 and 400 nm, respectively. Fitting results of the decay curves are listed in Table S4. The decay times both decrease with the increase of Mn2+ content, which further indicates the existence of energy transfer from Ce3+ to Mn2+. The calculated ηT values for cyanemitting Ce3+ to Mn2+ in CSAO:0.11Ce3+,xMn2+ are 29.4% (x = 0.01), 60.9% (x = 0.03), 72.6% (x = 0.05), and 80.9% (x = 0.07), while the ηT values for blue-emitting Ce3+ to Mn2+ in CSAO:0.11Ce3+,xMn2+ are 6.6% (x = 0.01), 22.4% (x = 0.03), 34.2% (x = 0.05), and 36.8% (x = 0.07). These ηT data indicate again that the energy transfer from cyan-emitting Ce3+ to Mn2+ is more efficient than that from blue-emitting Ce3+ to Mn2+. To better understand the mechanism of luminescence, an energy level diagram is shown in Figure 11, containing the excitation, emission processes of Ce3+ and Mn2+ in a CSAO host, and the energy transfer process from blue-emitting Ce3+ to cyan-emitting Ce3+ and from Ce3+ to Mn2+. It should be noticed that the 5d level of blue-emitting Ce3+ relates to the PLE band at ∼365 nm, while the two 5d levels of cyan-emitting

(4)

where τx and τ0 represent the lifetime of Ce3+ with and without Mn2+ in CSAO, which can be found in Table S3 and Table 2, respectively. The calculated ηT values for cyan-emitting Ce3+ to Mn2+ in CSAO:0.05Ce3+,xMn2+ are 17.3% (x = 0.01), 51.0% (x = 0.03), 72.4% (x = 0.05), and 83.7% (x = 0.07). The PL spectra of CSAO:0.11Ce3+,xMn2+ phosphors under excitation at 365 nm are shown in Figure 10a. The PL spectra contain the emission bands of blue- and cyan-emitting Ce3+ and red-emitting Mn2+, which are different from those of CSAO:0.05Ce3+,xMn2+ phosphors. The PL spectra excited at 300 nm as shown in Figure S3 are similar to that of CSAO:0.05Ce3+,xMn2+. Under excitation at 365 nm, the emission intensity of cyan-emitting Ce3+ decreases faster than that of blue-emitting Ce3+ with the increase in Mn2+ content. This result suggests that the energy transfer from cyan-emitting Ce3+ to Mn2+ is more efficient than that from blue-emitting Ce3+. This phenomenon may be due to the difference in spectral overlap. The cyan-emitting band of Ce3+ mainly overlaps the broad excitation band of Mn2+ (6A1 → 4T2 (4G)), while the blue-emitting band of Ce3+ mainly overlaps the narrow excitation band of Mn2+ (6A1 → 4A1/4E (4G)). Our previous work on Ca3Al2O6:Ce3+,Mn2+ also showed a similar phenomenon.33 The peak value of the red emission band shifts from 628 nm to 646 nm with increasing the content of Mn2+ from 0.01 to 0.07. The PLE spectra monitored at 620 nm are

Figure 11. Energy level diagram showing excitation, the emission process in Ce3+ and Mn2+ ions, and energy transfer from Ce3+ ions to Mn2+, together with the PLE and PL spectra of CSAO:0.11Ce3+ and CSAO:Mn2+. H

DOI: 10.1021/acs.inorgchem.6b02082 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Ce3+ correspond to the two overlapping PLE bands at around 300−330 nm. Figure 12 shows the chromaticity coordinates of CSAO:0.05Ce3+,xMn2+ and CSAO:0.11Ce3+,xMn2+ phosphors based on

85.2, respectively. While those for CSAO:0.11Ce3+,0.01Mn2+ are (0.272, 0.287) and 91.8, respectively. The CIE coordinates of these two phosphors are both near the blackbody locus, and the CRI values are both higher than 80. However, the CCT value for CSAO:0.11Ce3+,0.01Mn2+ is higher than 10 000 K, which may be due to a strong emission intensity in the purpleblue region. Better performance of white emission is expected in CSAO:Ce3+,xMn2+ with proper doping contents for both Ce3+ and Mn2+, where the blue emission of Ce3+ is lower than that of CSAO:0.11Ce3+,0.01Mn2+. Therefore, the selection of the CSAO:0.10Ce3+,xMn2+ series could also achieve better performance of CIE, CCT, and CRI, because CSAO:0.10Ce3+ exhibits an emission that has lower intensity in the purple-blue region than CSAO:0.11Ce3+ under excitation at around 365 nm. The PL spectra and photographs of CSAO:0.10Ce3+,xMn2+ excited at 365 nm are shown in Figure S4, and corresponding CIE, CCT, and CRI values are also listed in Table S5 in the Supporting Information. The results suggest that the CCT value decreases indeed with CRI maintaining a high value. Finally, a thermal stability analysis is performed. The temperature-dependent PL spectra of CSAO:0.05Ce3+,0.02Mn2+ are shown in Figure S5. The results indicate that the thermal stability is not good upon heating, which needs to be further improved.

Figure 12. Evolution of CIE chromaticity coordinates for CSAO:0.05Ce3+,xMn2+ and CSAO:0.11Ce3+,xMn2+ and their corresponding photographs excited at 365 nm.

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the PL spectra excited at 365 nm. It is clear that the color point evolves from the cyan region to the orange region for the CSAO:0.05Ce3+,xMn2+ series, while the color point shifts from the cyan region to the purplish-red region for the CSAO:0.11Ce3+,xMn2+ series. The white region is passed with the increase of Mn2+ content for both series. The photographs of the phosphors under a 365 nm near-UV lamp are also illustrated in Figure 12, which exhibit a tunable color emission intuitively. The International Commission on Illumination coordinates (CIE), correlated color temperature (CCT), and color rendering index (CRI) of CSAO:0.05Ce3+,xMn2+ and CSAO:0.11Ce3+,xMn2+ phosphors are listed in Table 3. They are worth noticing for CSAO:0.05Ce3+,0.02Mn2+ and CSAO:0.11Ce3+,0.01Mn2+. The CIE coordinate and CRI values of CSAO:0.05Ce3+,0.02Mn2+ are (0.337, 0.352) and

CONCLUSIONS Ca2.5Sr0.5Al2O6:Ce3+,Li+ phosphors with multicolor emission and white-emitting single-composition Ca2.5Sr0.5Al2O6:Ce3+,Li+,Mn2+ phosphors with high CRI values are fabricated by a high-temperature solid-state reaction. The relationship between emission bands and crystallographic sites for Ce3+ was studied by crystal structure refinement and PL, PLE, and decay curve measurements. Site-sensitive energy transfer was observed in Ca2.5Sr0.5Al2O6:Ce3+,Li+,Mn2+ phosphors, which is due to the different manners of spectral overlap. Three kinds of Ce3+, Mn2+ codoping ways were performed based on different PL profiles of Ce3+ under excitation at around 360 nm with different Ce3+ contents. The emission color of codoped phosphors can be tuned from cyan to white and then to red, and single-phase white-emitting phosphors with high CRI values were achieved. Studies on the properties of white-emitting phosphors indicate that these phosphors are promising candidates for near-UV-excited w-LEDs.

Table 3. CIE Coordinates and CCT and CRI Values of CSAO:0.05Ce3+,xMn2+ and CSAO:0.11Ce3+,xMn2+ Calculated Based on the PL Spectra Excited at 365 nm

phosphor 3+

CSAO:0.05Ce ,xMn

2+

CSAO:0.11Ce3+,xMn2+

x x x x x x x x x x x x x x x x

= = = = = = = = = = = = = = = =

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07

CIE coordinates (x, y)

CCT (K)

CRI values

0.258, 0.266, 0.337, 0.369, 0.397, 0.420, 0.450, 0.469, 0.236, 0.272, 0.308, 0.334, 0.353, 0.379, 0.410, 0.394,

9024 8755 5315 4156 3340 2801 2184 2027 14 379 10 924 7325 5369 3967 2708 1961 1853

69.0 74.3 85.2 75.1 68.6 66.0 63.6 62.8 74.2 91.8 69.1 51.6 42.5 43.1 46.1 43.3

0.356 0.355 0.352 0.350 0.349 0.348 0.346 0.345 0.297 0.287 0.288 0.277 0.266 0.268 0.272 0.248

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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.6b02082. SEM images, atomic parameters, PL and PLE spectra, fitting results, CIE coordinates, and CCT and CRI values (PDF) CIF file of CSAO:0.05Ce3+ (CIF) CIF file of CSAO:0.11Ce3+ (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J. Zhang). *E-mail: [email protected] (S. Lian). ORCID

Jilin Zhang: 0000-0001-7235-341X I

DOI: 10.1021/acs.inorgchem.6b02082 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is partially supported by the National Natural Science Foundation of China (grant nos. 51402105, 21571059, 21471055, 21501058), National Key Research and Development Program (grant no. 2016YFB0302403), and Specialized Research Fund for the Doctoral Program of Higher Education (grant no. 20134306120009).

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K

DOI: 10.1021/acs.inorgchem.6b02082 Inorg. Chem. XXXX, XXX, XXX−XXX