Redistribution of Activator Tuning of Photoluminescence by Isovalent

Jun 30, 2015 - Key Laboratory of Chemical Biology and Traditional Chinese Medicine Research (Ministry of Education of China), Key Laboratory of Sustai...
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Redistribution of Activator Tuning of Photoluminescence by Isovalent and Aliovalent Cation Substitutions in Whitlockite Phosphors Jin Han,† Wenli Zhou,*,† Zhongxian Qiu,† Liping Yu,† Jilin Zhang,† Qingji Xie,† Jing Wang,‡ and Shixun Lian*,† †

Key Laboratory of Chemical Biology and Traditional Chinese Medicine Research (Ministry of Education of China), Key Laboratory of Sustainable Resources Processing and Advanced Materials of Hunan Province College, College of Chemistry and Chemical Engineering, Hunan Normal University, Changsha 410081, China ‡ School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou 510275, China S Supporting Information *

ABSTRACT: Many strategies, including double substitution, addition of charge compensation, cation-size-mismatch and neighboring-cation substitution, have contributed to tuning photoluminescence of phosphors for white light-emitting diodes. These strategies generally involve modification of a certain special site where the activator occupies; tuning strategy based on multiple cation sites is very rare and desirable. Here we report that isovalent (Sr2+) and aliovalent (Gd3+) substitutions for Ca2+ tune the photoluminescence from one band to multiple bands in whitlockite β-Ca3−xSrx(PO4)2:Eu2+ and βCa3−3y/7Gd2y/7(PO4)2:Eu2+ phosphors. The saltatory variation of the emission spectra is caused by the removal of Eu2+ from the site M(4) to other sites. Moreover, we found the mechanisms of dopant redistribution tuning the luminescence are different. The incorporation of Gd3+ makes the site M(4) empty according to the scheme 3Ca2+ = 2Gd3+ + □, while Sr2+ substitution causes the cation sites to be enlarged due to cation size mismatch. Additionally, the influence of the cation substitutions on the photoluminescence thermal stability of phosphors is researched. The strategies, emptying and enlarging sites, developed herein are expected to provide a general route for tuning luminescence of phosphors with multiple sites in the future. substitution of Mg2+−Si4+/Ge4+ in YAG:Ce3+ garnet phosphor leads to an obvious red shift of emission wavelength under the excitation of blue light.9 Addition of charge compensation into another garnet Ca3Sc2Si3O12:Ce3+, Mn2+ produces a tunable full-color emission.10 Cation-size-mismatch, neighboring-cation substitution and release of neighboring-cation-induced stress recently have been proposed to tune photoluminescence and thermal quenching. 11−14 In addition, two iso-structural compounds as end-members composition forming solid solution could control the photoluminescence of phosphors.15,16 However, these strategies only involve modification of a certain special site where the luminescence center ion occupies. When there are more than one cation sites in a host lattice, the dopant ion could preferentially enter a more appropriate site because of a possible size mismatch between the luminescent ion and the host cation at the specific site, leading to a high energy emission. In general, the amount of dopant in phosphors is small; increasing concentration will cause luminescence quenching. Very recently, an ingenious approach, a high concentration doping forces the luminescence

1. INTRODUCTION Novel phosphors with Ce3+ and Eu2+ ions as activators for solid state lighting have attracted much attention due to the photoluminescence tunability over the whole visible spectrum.1−4 Although the intrinsic characteristics of the activator ions contribute to the luminescence properties of phosphor, the intrinsic characteristics are determined by the host lattice characteristics. Two major effects dictate the luminescence properties of the activators in a host lattice: centroid shift and crystal field splitting. The centroid shift is linked to the polarizability of the surrounding anion ligands and to the covalency of the crystal.5 The magnitude of crystal field splitting depends on the bond lengths from the activator ion to the coordinating anions, the degree of covalency between the activator ion and its ligands, the coordination environment, and the symmetry of the activator site.6,7 However, it is still a great challenge to fundamentally understand the relationship between the luminescence of phosphors and host lattice. The excitation and emission spectra, even the thermal quenching and thermal degradation behaviors of phosphors can be tuned through the host composition and structure. The substitutions of Si4+ by Al3+ and Ge4+ cause the redistribution of Ce3+ on two distorted octahedral Ca sites of γ-Ca2SiO4:Ce3+.8 As a result, the blue luminescence is shifted to yellow. A double © XXXX American Chemical Society

Received: May 26, 2015 Revised: June 28, 2015

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DOI: 10.1021/acs.jpcc.5b04997 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 1. Schematic structures of the β-Ca3(PO4)2 and Ca-centered polyhedra.

stitution20,21 with interesting results. Therefore, we select the whitlockite-type β-Ca3(PO4)2:Eu2+ as a model phosphor to investigate the redistribution of Eu2+ among multi-Ca sites by introducing Gd3+ and Sr2+ based on charge and size mismatch, respectively.

ion to enter a less appropriate site, was proposed to selectively tune a desired emission color in a phosphor with multication sites.2 Deep red emission appears at the cost of yellow emission, which is due to concentration quenching. Although it is a new strategy to tune photoluminescence, the concentration quenching method causes the cost of phosphors to increase. Moreover, it is difficult to systematically tune the photoluminescence over the full visible spectrum. Inspired by these, we assume, in a multiple site phosphor [for example, β-Ca3(PO4)2], substitutions of high valent (Gd3+) or large isovalent (Sr2+) ion for host cation (Ca2+) possibly facilitate the migration of dopant (Eu2+) from one site to another. The former requires some cation vacancies to be present, resulting in the dopant moving from the site, where the vacancies are produced, to other sites. For the latter, these host cation sites will be enlarged due to the large ion substitution (Sr2+ for Ca2+), possibly causing more appropriate sites for the dopant to occupy. As a consequence, the redistribution of dopant among crystallographic independent sites could control photoluminescence of the phosphor. Whitlockite phosphors have received considerable attention. Huang et al. tuned white light emission in a single phase Ca9Y(PO4)7:Eu2+, Mn2+ phosphor based on energy transfer.17 Xia et al. identified the crystallographic sites of Eu2+ in Ca9NaMg(PO4)7 structure and studied the luminescence properties.18 Ji et al. recently reported a yellow phosphor Sr1.75Ca1.25(PO4)2:Eu2+.19 Due to the special structural features of whitlockite phosphors, some groups researched the photoluminescence behavior by cation sub-

2. EXPERIMENTAL SECTION 2.1. Materials and Synthesis. The β-Ca3−xSrx(PO4)2:Eu2+ (TCPESx, 0 ≤ x ≤ 1.5), β-Ca 3−3y/7 Gd 2y/7 (PO 4 ) 2 :Eu 2+ (TCPEGy, 0 ≤ y ≤ 1.0), and β-Ca 3−z Mg z (PO 4 ) 2 :Eu 2+ (TCPEMz, 0 ≤ z ≤ 0.3) solid solutions were fabricated by conventional solid-state reaction. The concentration of dopant Eu2+ is fixed at 0.8 mol %. The starting materials, CaCO3 (AR), SrCO3 (AR), NH4H2PO4 (AR), MgO (AR), Gd2O3 (99.99%), and Eu2O3 (99.99%), with stoichiometric molar ratios, were thoroughly ground and mixed in an agate mortar. The mixtures were preheated at 1000 °C for 8 h in air, sintered at 1100 °C for 10 h under a reducing atmosphere of 15% H2/85% N2, and finally slowly cooled to room temperature. 2.2. Characterization. The phase purity of all samples was examined by X-ray diffraction (XRD) using a Bruker D8 ADVANCE powder diffractometer with Cu Kα radiation (λ = 1.54059 Å). Diffuse reflectance spectra (DRS) were taken on a Cary 5000 UV−vis−NIR spectrophotometer. Raman spectra were collected by DXR Raman microscope. The photoluminescence excitation (PLE), photoluminescence (PL) spectra, and fluorescence decay curves were recorded on an Edinburgh FSL920 time-resolved and steady-state fluorescence B

DOI: 10.1021/acs.jpcc.5b04997 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C spectrometer, which is equipped with a time-correlated singlephoton counting card and a thermoelectric cooled red sensitive photomultiplier tube. A 450 W xenon lamp is used as the excitation source. The excitation photons for the fluorescence decay curves are provided by a 150 W nF900 flash lamp. The sample temperature is varied by means of a temperature controller (Oxford, CRY TEMP).

3. RESULTS AND DISCUSSION 3.1. Crystal Structure and Phase Identification. Previous studies have reported the crystal structure of the whitlockite β-Ca3(PO4)2 (TCP),22−24 as shown in Figure 1. There are five crystallographic independent cation sites (M) in TCP, each of the M(1), M(2), M(3), and M(5) sites are fully occupied by one Ca atom. On the basis of the Ca−O bond lengths ( 0.6. For higher Sr compositions, the violetblue emission band nearly disappears, and the emission spectra mainly cover the green and red region. Therefore, the luminescence of the TCP:Eu2+ phosphor can be tuned by the simple incorporation of Sr2+. Recently, Ji et al. also reported the cation substitution dependent photoluminescence in β-Ca3−xSrx(PO4)2:Eu2+.21 They considered the Eu2+ evenly occupy the four M sites [M(1)-M(4)] of TCP and suggested that the incorporation of Sr2+ would vary the external environment of EuOn polyhedra in TCP:Eu2+. As a result, a conclusion was obtained that the emission bands could be related to the EuOn−Ca9−xSrx emitting blocks. However, it is difficult to give an explanation as to why the PL peak jumps suddenly from 418 to 493 nm, as said in the paper.21 In fact, lots of papers have reported the study on the preferred sites of the activators in phosphors with multiple sites.27 For all this, we suppose that the saltatory variation is caused by Eu2+ occupying different cation sites in the single phase phosphor.28,29 For the no Sr sample, only one violet-blue emission band can be observed. However, when replacing Ca2+ with Sr2+, extra emission bands appear successively. The Eu2+ concentration is constant for all the samples, so one can image that introducing Sr2+ causes the redistribution of Eu2+ among different M sites in the TCPESx host lattice. Additionally, Chen et al. recently observed two

= 0, 0.3, 0.9, and 1.5 at RT. All decay curves consist of both fast and slow components and are fitted with a double-exponential decay function. The concentration dependence of the average lifetimes is displayed in Figure S1 of the Supporting Information. When excited at 300 nm and monitored at 400 nm, the decay time of Eu2+ quickly decreases from 508 ns (x = 0) to 3 ns (x = 1.5), indicating an extra channel for the excited Eu2+ to depopulate. When excited at 354 nm and monitored at 490 nm, the decay curves of the Eu2+ center show a small variation (Figure 6b). The average decay time increases at first and then decreases (Figure S1), suggesting that an efficient energy transfer among Eu2+ centers could occur. These decay curves further prove the affirmation for existing multiple Eu2+ centers in Sr-substituted samples. If the emission around 418 nm is only originated from one site, the question becomes which site preferentially accommodates the dopant Eu2+? First, site M(5) is exclusively occupied by Ca due to its small polyhedron volume (14.71 Å3) and very short Ca−O bond length (2.26 Å).24 Furthermore, we note that some papers have reported the photoluminescence of Eu2+ doped (Ca,Mg,Sr)9Y(PO4)7 and Ca9Y(PO4)7 phosphors,17,31 the structure of which is the same as β-Ca3(PO4)2. Both the phosphors emit blue-green light around 490 nm, which is very similar to one of the emission bands of asprepared TCPESx samples. So, we considered incorporating Gd3+ to replace the partial Ca2+ of TCP based on the close ion radii and charge mismatch. As expected, the violet-blue emission decreases, and the cyan emission appears and contrarily increases with increasing the concentration (y) of Gd3+ (Figure 7a). The excitation spectrum is enhanced (Figure S2 of the Supporting Information) and broadened after D

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M(4) site is the loosest one for Eu2+ to accommodate among the five sites. The introduction of Gd3+ and Sr2+ ions causes the saltatory variation for the emission spectra, indicating some Eu2+ centers migrated from the loose M(4) site to other tight sites, for instance, M(3), M(2), and M(1). Additionally, we carried out the substitution of La3+ for Ca2+, the PL spectra show a similar result as that of Gd3+ replaced samples, as shown in Figure S3 of the Supporting Information. The emission spectra of TCPESx and TCPEGy samples with intermediated x and y values consist of multiple sub-bands. Taking the emission spectrum of TCPEG0.8 as an example, the PL profile can be fitted into four sub-bands peaking around 419, 483, 529, and 600 nm (Figure 7b). Considering the average Ca−O bond lengths (l) at these sites, lM(3) > lM(2) > lM(1),24 the last three fitted sub-bands should originate from Eu2+ ions at site M(3), M(2), and M(1), respectively. L.G. Uitert34 pointed out the position of the d-band edge (E) in energy for Eu2+ ions is related to some parameters, including the coordination number (n), electron affinity (ea), radius (r), and valence (V) of the host cation and the position in energy (Q = 34000 cm−1) for the lower d-band edge for the free ion. He even proposed an empirical relation 1 to predict the emission peak and/or excitation edge data for Eu2+ ion in a compound.

Figure 7. (a) Emission spectra of TCPEGy phosphors, (b) Gaussian fitting of emission spectrum of TCPEG0.8. The inset shows the excitation spectra of TCPE (λem = 418 nm) and TCPEG1.0 (λem = 493 nm) samples. 3+

E = Q × [1 − (V /4)1/ V × 10−(n × ea × r)/80]

2+

introducing Gd into TCP:Eu (see the inset of Figure 7b). The variation is similar to that of the Sr-incorporated samples. Raman technique is an efficient tool for investigating the structure of compounds containing a lot of structural defects.32 Figure 8 shows the typical Raman spectra of TCPEGy. The

(1)

In the TCP host, the ea of Ca atom is 1.95; for site M(4), n = 3, r = 1.00 Å, and V = 0.7; for M(3), n = 8, r = 1.12 Å, and V = 1.8; for M(2), n = 8, r = 1.12 Å, and V = 2.1; and for M(1), n = 7, r = 1.06 Å, and V = 2.0.24 The four E values are calculated to be 31618, 20805, 18871, and 18147 cm −1 , and the corresponding emission wavelengths are about 416, 481, 530, and 551 nm, respectively, which are quite close to the experimental values (419, 483, 529, and 600 nm). 3.3. Redistribution Mechanism of Dopant Eu2+. In summary, this study has proved that the redistribution of dopant Eu2+ tunes the photoluminescence via two strategies, aliovalent substitution of Gd3+ and isovalent substitution of Sr2+ for Ca2+ in TCP. For the former, the incorporation of Gd3+ introduces vacancies at site M(4) and then forces the Eu2+ ions to remove from this site to other sites. Here we call it as “emptying site” effect. For the latter, Sr has two roles: one is enlarging the cell parameters due to cation size-mismatch, especially enlarging the M(3), M(2), and M(1) sites and making these sites capable for Eu2+ to accommodate; another is competing with Eu2+ at the loose M(4) site due to the extremely similar ion radii. Moreover, when Ca2+ are substituted by smaller Mg2+ ions, the emission spectra only show a continuously weak blue shift from 418 to 411 nm with increasing Mg2+ content (z ≤ 0.3) (Figures S4 and S5 of the Supporting Information). It indicates that smaller Mg2+ cannot drive the removal of Eu2+ ions from the M(4) site to the other sites. So we name the substitution of Sr2+ as the “enlarging site” effect. The redistribution mechanism of Eu2+ induced by both the effect are proposed and shown in Scheme 1. Although Gd3+ shares the positions M(1), M(2), and M(3) with Ca2+, the partial substitution of Gd3+ does not cause the host lattice expansion due to its ion radius being a little smaller than Ca2+. Therefore, Eu2+ mainly redistributes into 8-coordinated M(3) and M(2) sites for the incorporation of Gd 3+ ; the corresponding PL dominates at about 490 nm for high Gd3+ doping concentration. However, for high Sr2+ incorporation, besides the M(3) and M(2), Sr2+ also enlarges and occupies the

Figure 8. Raman spectra of as-prepared TCPEGy samples.

Raman lines are located in four distinctly separate ranges, 1005−1094 cm−1 (υ3), 940−970 cm−1 (υ1), 545−634 cm−1 (υ4), and 403−476 cm−1 (υ2), which correspond to the internal modes. The intensity of the Raman line at 948 cm−1 from internal vibrations (υ1) of the PO43− ions decreases with increasing y in the TCPEGy phosphates, indicating the formation of vacancies at the M(4) positions due to the replacement of Ca2+ ions by Gd3+ ions according to the scheme 3Ca2+ = 2Gd3+ + □.33 Together with emission spectra of TCPEGy, we are able to arrive at a conclusion that the emission around 418 nm is from Eu2+ at the site M(4). From the point of Ca(4)O9 polyhedron (see Figure 1), the conclusion is also reasonable, because the E

DOI: 10.1021/acs.jpcc.5b04997 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C Scheme 1. Redistribution of Dopant Eu2+ among Different Sites Driven by (a) Emptying Site and (b) Enlarging Sites in Whitlockite Phosphor Lattices

Gd3+ cause a worse thermal stability, which may be related to the distance between activator and vacancy (□) at the site M(4). The shortest distance of M(4)-M(4) is about 18.70 Å, M(3)-M(4) is 3.64 Å, M(2)-M(4) is 5.01 Å, and M(1)-M(4) is 4.76 Å. For the TCP:Eu phosphor, the activator Eu2+ only locates at the loose M(4) site and has a distance of at least 18.70 Å to the vacancies. After some Sr2+ or Gd3+ are introduced, the Eu2+ would migrate from the M(4) to M(3), M(2), or even the M(1) site, resulting in the significant shortening of the distance between activator and vacancy. Therefore, when increasing the content of Sr2+ or Gd3+, the PL thermal stability became worse due to the shortened distance. Additionally, the substitution of Gd3+ for Ca2+ causes more vacancies at M(4) sites due to charge mismatching. So, the Gd3+-rich phosphor, especially for y = 1, could show the worst thermal stability.

tight site M(1), more Eu2+ would distribute into this site and lead to the emission band dominated around 600 nm. 3.4. Influence of Cation Substitution on the PL Thermal Stability. The PL thermal stability is one of the most important parameters of phosphor used in LEDs. It closely relates to the crystal lattice environment of the activator. Accordingly, we tested the temperature-dependent luminescence spectra of Sr2+ (x = 0, 0.3, and 0.9) and Gd3+ (y = 0, 0.6, and 1.0) substituted samples in air from 300 to 500 K. The normalized PL intensity as a function of the temperature is plotted in Figure 9. The pure TCP:Eu sample (x = 0 or y = 0)

4. CONCLUSIONS A simple solid-state reaction route was adopted to fabricate the whitlockite solid solution phosphors, β-Ca3−xSrx(PO4)2:Eu2+ and β- Ca3−3y/7Gd2y/7(PO4)2:Eu2+. The 418 nm emission band of β-Ca3(PO4)2:Eu2+ phosphor is confirmed to be from Eu2+ on the site M(4). The emission color of the phosphor can be tuned by incorporating isovalent Sr2+ and aliovalent Gd3+ ions. In this work, we clearly demonstrated that the luminescence tunability is related to the redistribution of dopant Eu2+ after the incorporation of Sr2+ or Gd3+. The redistribution mechanisms of Eu2+ are proposed as emptying and enlarging site effects. For the former, incorporating Gd3+ causes the M(4) to empty due to charge mismatch and forces the Eu2+ to remove from this site to other sites. For the latter, incorporating Sr2+ enlarges the cation sites and makes the M(3), M(2), and M(1) sites more appropriate for Eu2+ to accommodate. On the basis of both effects, we believe that luminescence of more phosphors with multiple sites could be tuned for solid state lighting in the future.

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ASSOCIATED CONTENT

S Supporting Information *

Decay time of Eu2+ ions, PL spectra of La3+-replaced samples, and XRD and emission spectra of Mg-incorporated samples. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b04997.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Tel/Fax: +86-731-88865345. Notes

The authors declare no competing financial interest.

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Figure 9. Thermal stability of photoluminescence for (a) TCPESx and (b) TCPEGy samples.

ACKNOWLEDGMENTS This work is partially supported by the Aid Program for Science and Technology Innovative Research Team in Higher Educational Institutions of Hunan Province, the National Natural Science Foundation of China (Grants 51402105 and 21471055), Hunan Provincial Natural Science Foundation of China (Grant 2015JJ2100), the Hunan Planned Projects for Postdoctoral Research Funds (Grant 2013RS4025), and Postdoctoral Science Foundation of China (Grant 2013M542117).

shows the best PL thermal stability among all of the samples, the relative PL intensity at 440 K is about 85% of the initial intensity at 300 K. After introducing Sr2+ of 0.3, the thermal stability gets worse. When increasing the concentration of Sr2+ up to x = 0.9, the decreasing tendency of PL intensity further increases with increasing the tempereture (Figure 9a). The incorporation of Gd3+ (y = 0.6) also harms the PL thermal stability, the sample with y = 1.0 even shows the worst thermal stability (Figure 9b). It is interesting that introducing Sr2+ and F

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DOI: 10.1021/acs.jpcc.5b04997 J. Phys. Chem. C XXXX, XXX, XXX−XXX