HE Xio-Wu LIU Xio-Fng LI Jin-Rong YU Rong-Hi

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Local Structure Mediation and Photoluminescence of Ce3+- and Eu3+-Codoped YAG Nanophosphors①

HE Xiao-WuaLIU Xiao-FangaLI Jian-Rongb②YU Rong-Haia②

a(100191)b(350002)

Cerium and europium codoped yttrium aluminum garnet (YAG:Ce,Eu) nano- phosphors were prepared by sol-gel method. We systematically explored the structure, composition, morphology and photoluminescence (PL) properties by using X-ray diffraction, scanning electron microscope, high resolution transmission electron microscope, energy dispersive spectrometer, photoluminescence emission and excitation spectra techniques, and focused on clarifying the change of local structure surrounding Ce3+ions by utilizing advanced quantitative27Al magic angle spinning nuclear magnetic resonance spectroscopy. The results show that the lattice constant slightly increases as the Ce3+and Eu3+ions incorporate, and the geometric distortion of local structure surrounding Ce3+activator introduced by the incorporated Eu3+coactivator causes the variation of crystal field, which results in red shift of Ce3+PL emitting in YAG:Ce,Eu nanophosphor. Furthermore,the YAG:Ce,Eu nanophosphors could exhibit several sharp and narrow50→7J(= 1～4) emissions of Eu3+ion besides the classic broad 51→ 4(25/2,27/2) emissions of Ce3+ion under near ultraviolet (UV) excitation.

YAG:Ce,Eu nanophosphor, photoluminescence, crystal structure, WLED;

1 INTRODUCTION

As a classic down-conversion material, cerium-do- ped yttrium aluminum garnet (Ce3+-doped Y3Al5O12, YAG:Ce) displays strong absorption of blue excitation radiation, high quantum efficiency, short decay and favorable stability[1-3]. YAG:Ce has attracted intensive attention in exploiting prominent photoluminescence (PL) materials since Nakamura et al.[4]assembled the first blue light-emitting diode (LED) via combining blue LED chip with yellow YAG:Ce phosphors[5]. Nevertheless, YAG:Ce phos- phors still suffer from a poor color-rendering index (CRI) due to the lack of color in red-emitting region produced by red components[6]. To solve this problem, a portion of prospective activators was introduced into phosphors for an enhanced red component. So far, several materials such as (Ca, Sr)S:Eu2+, Y2(MoO4)3:Eu3+and SrTiO3:Pr3+have been applied as red phosphors for white LED (WLED)[7,8], but their chemical stabilities are not satisfactory. Attractively, incorporating trivalent rare earth (RE3+) ions as coactivators into YAG:Ce phosphors becomes effective to improve the red emitting of YAG:Ce phosphors, because these codopants can act as not only coactivators but also wavelength shifters while their similar ionic radii and electronegativity as that of substituted ions won’t change the structure of YAG[9,10]. Among various tri-valence metal ion (ME3+) codoped YAG:Ce[11], the Eu3+-codoped YAG:Ce (YAG:Ce,Eu) is a proposing candidate for red phosphors with excellent yellow and red PL emitting[12], because Eu3+ion has narrow emission band and long fluorescence lifetime of optically active states.

As we know, both the Ce3+and Eu3+ions could easily replace partial dodecahedral Y3+ions as coactivatorsand do not change the2point symmetry, whereas they would induce a geometric distortion of symmetry into the local structure of YAG[13]. Furthermore, the sensitivity of 5energy levels to the local structure of Ce3+and Eu3+ions can give rise to the nephelauxetic effect (covalency), crystal-field splitting and Stokes shift[14]. On the other hand, the incorporated lattice site and the local environment of doped ions could cause structural and chemical defects which significantlyinfluence the PL properties of YAG:Ce,Eu[15].Despite that not a few researchers have studied the crystal structures and the related PL properties of YAG:Ce and/or YAG:Eu, respectively[3,16], the intrinsic relationship between the PL properties and local structure of YAG:Ce,Eu is rarely studied. Lu et al.[17]characterized the crystal structure and PL properties of YAG:Eu nanophos- phors obtained by sol-gel method. Cheetham et al.[18,19]investigated the relationship between PL properties and average crystal structure of YAG:Ce by synchrotron powder diffraction. Hence, it is worth noting that understanding the variations of local structureof YAG after Ce3+and Eu3+ions incur- poration are fundamental and crucial for improving the red emitting of YAG:Ce,Eu nanophosphors.

In contrast to the previous characterization techniques[20,21], high-resolution solid-state nuclear magnetic resonance (NMR) spectroscopy is parti- cularly adept at analyzing the local structure and coordination state of NMR activated nuclear from atom/molecular level because of the high sensitivity of NMR chemical shift towards the local chemical environment of relevant nucleus[22-24]. Thus, this technique is potently used in examining the local structure and the correlated small distortion surrounding Ce3+and Eu3+ions in YAG:Ce,Eu nanocrystals since it has supplied a novel access for acquiring quantitative results from27Al magic angle spinning (MAS) NMR spectra of quadrupolar nuclei in different local environments. In the present work, a feasible sol-gel method was used to prepare Ce3+- and Eu3+-codoped YAG nanophosphors. The crystal structure, composition, morphology and PL properties of YAG:Ce,Eu nanophosphors were systematically studied by X-ray diffraction, scanning electron microscope, high resolution transmission electron microscope, energy dispersive spectrometer, photoluminescence emission and excitation spectra techniques. Peculiarly, we used the quantitative high-resolution solid-state27Al MAS NMR technique to precisely clarify the small distortion of the local environment surrounding Ce3+and Eu3+coactivators in YAG nanocrystals. This study is helpful for designing and equipping the high-quality red YAG:Ce,Eu nanophosphors.

2 EXPERIMENTAL

2. 1 Syntheses

(Y1-x-y,Ce,Eu)3Al5O12(YAG:Ce,Eu) sample was synthesized by modified sol-gel method. Each reagent was of analytical grade and used without further purification. Citric acid (CA) and ethylene glycol (EG) were used as chelating agents. Y(NO3)3·6H2O, Ce(NO3)3·6H2O, Eu(NO3)3·6H2Oand Al(NO3)3·9H2O were employed as cation sources. The nitrate salts were dissolved in 120 mL deionized water with stoichiometric ratio of 3×(1 ––):3×():3×():5 withequaling to 0.0055 andequaling to 0.0055 (0.0011 and 0.0550), and then CA and EG were poured into the solution. The solution was continuously stirred at 50 ℃for evaporating overmuch water and accelerating polyesterification schedule, and then the gels were heated in an oil-bath at 100 ℃ for 24 h. Finally, the precursors were ground and calcined at 930 ℃ for 3 h in air. Additionally, YAG and YAG:Ce(= 0.0055) powders were also prepared by the same method in rigid stoichiometric ratio.

2. 2 Characterizations

Crystal structures of YAG, YAG:Ce and YAG:Ce,Eu samples were characterized by powder X-ray diffraction (XRD) on the Rigaku D/max-2500 diffractometer using Curadiation filtered by graphite with the experimental parameters of 40 kV, 200 mA, and 2 °·min-1.Then Rietveld refinements were performed on the collected data by using Jade 5.0 software to get the cell parameters and average crystal sizes. The27Al high-resolution solid-state NMR spectra of the YAG, YAG:Ce and YAG:Ce,Eu samples were acquired at 104.2 MHz on a Bruker 400M Avance III HD NMR spectrometer (9.4 T) equipped with a 4 mm H/X CP MAS NMR probe at a spinning rate of 12 kHz. The quantitative27Al MAS NMR spectra were measured with a 0.14 μs 9opulse width, corresponding to 4.2 μs 90opulse width of 1 M Al(NO3)3solution and the recycle delay of 1 s. The morphology of samples was observed by a JEOL JSM-7500F scanning electron microscope (SEM). The microstructure and composition were measured on a JEOL JEM-2100F transmission electron microscope (TEM) equipped with an Oxford energy dispersive spectrometer (EDS).The PLE and PL emission spectra of YAG:Ce,Eu nanophosphors were acquired on a Hitachi F-4600 fluorescence spectro- photometer using a 150 W Xenon short-arc lamp as excitation source (wavelength range: 200～900 nm, excitation slit: 2.5 nm, emission slit: 2.5 nm, PMT voltage: 700 V).

3 RESULTS AND DISCUSSION

3. 1 Crystal structure

The crystal structures of pure and doped YAG samples were examined by XRD measurements and exhibited in Fig. 1. Fig. 1(a) shows the XRD patterns of YAG, Ce3+-doped YAG and Ce3+- and Eu3+-codoped YAG samples sintered at 930 ℃ for 3 h. The XRD curve of the YAG:Ce,Eu sample exhibited similar diffraction peaks with that of YAG and YAG:Ce samples, which could be faultlessly indexed to the cubic structure of Y3Al5O12(JCPDS: 33-0040)[25]. No impure diffraction peaks could be detected, indicating that the incorporated Ce3+and Eu3+ions were promised to enter the lattice sites without changing the crystal framework of YAG phase. It implies that the pure YAG phase can be successfully acquired and no other crystalline phases such as Y4Al12O9(YAM) or YAlO3(YAP) form via sol-gel method, and the related calcined temperature was lower than that for the traditional solid-state methods (＞1500 ℃). Moreover, we further analyzed the crystal structure of YAG:Ce,Eu with different Eu3+doping concentration (shown in Fig. 2), and the results showed that the Eu3+-doped con- centration did not change the nanocrystal outline of YAG structure.

Table 1. Lattice Parameters of YAG, YAG:Ce and YAG:Ce,EuSamples

Fig. 1. XRD patterns of YAG, YAG:Ce and YAG:Ce,Eu nanocrystals sintered at 930 ℃in the 2scan ranges of (a) 10～70oand (b) 32～34o

Fig. 2. XRD patterns of YAG:Ce0.0055, Eu(= 0.0011, 0.0055, 0.0550) nanophosphors sintered at 930 ℃

3. 2 Coordinating environment of Al3+ surrounding the Ce3+ and Eu3+ ions

Solid-state NMR spectroscopy is good at recog- nizing the different phases and coordination states of NMR activated nuclei-containing material because of the high sensitivity of NMR chemical shifts towards the fine chemical environment of nucleus, especially for the material with short range order where powder XRD can supply little information. Fig. 3 provides the27Al MAS NMR spectra of YAG, YAG:Ce and YAG:Ce,Eu nanocrystals sintered at 930 ℃. A sharp and narrow line at 1.2 ppm is observed for all of the three samples, which is assigned to the octahedral (six-coordinate) AlO6species[28].Another weak signal at –16.7 ppm appearing at27Al MAS NMR spectra of YAG:Ce and YAG:Ce,Eu nanophosphors is located in the characteristic chemical shift range of AlO6species as well[29,30]. From the relative intensity of the peaks at 1.2 and –16.7 ppm, it is concluded that the weak line at –16.7 ppm arises from the AlO6units in the first coordination sphere of paramagnetic Ce3+or Eu3+ions in nanophosphors, which is further supported by the absence of peak at –16.7 ppm in the spectrum of the YAG without doping Ce3+and Eu3+ions (Fig. 3(a)) and keeps accordance with the previous literature[22]. Its chemical shift to higher field relative to the typical AlO6species (1.2 ppm) is caused by the adjacent Ce3+and/or Eu3+ions whose unpaired electrons in the 4shell influence the local magnetic field of adjacent Al nuclei. When a paramagnetic substance is placed in a magnetic field, a net magnetization is induced in the substance. This magnetization produces a new magnetic field that the probed atom or isotope feels in addition to the magnetic field produced by the spectrometer. This causes an isotropic shift of atoms in the vicinity of the paramagnetic substance.

Fig. 3.27Al MAS NMR spectra of (a) YAG, (b) YAG:Ce and (c) YAG:Ce,Eu samples. All the samples were sintered at 930 ℃. Spin rate was 12 kHz and the asterisks stand for spinning bands

It was clear that the line width of peak at 1.2 ppm was broadened after Ce3+doping and Ce3+, Eu3+codoping into YAG nanocrystal. The peak broade- ning effect could be explained by the presence of paramagneticCe3+and Eu3+ions in close proximity of AlO6units. Moreover, the line width at 1.2 ppm in YAG:Ce,Eu nanophosphors was broader compared with that of YAG:Ce nanophosphors, indicating that more Y3+ions were further substituted by Eu3+ions in Ce3+- and Eu3+-codoped YAG nanophosphors than in single Ce3+-doped YAG nanophosphors, which is further supported by the enhanced peak intensity at –16.7 ppm in the27Al MAS spectrum of YAG:Ce,Eu nanophosphors compared with that of YAG:Ce nanophosphors.It was evidenced by experimental[31]and theoretical[32]work that the geometric distortion of the centrosymmetric2hpoint symmetry of the dopant ions could cause the change of crystal field strength, which resulted in the crystal field splitting of the 5level of Ce3+ions and further generated redshift or blueshift in codoped YAG:Ce phosphors. Herein, both the radii of Ce3+and Eu3+ions are larger than that of Y3+ion, but the radius of Eu3+is closer to that of Y3+than Ce3+ion. Therefore, we proposed that the incorporated Eu3+would produce small disturbance to the local structure around Ce3+activa- tor in Ce3+- and Eu3+-codoped YAG nanophosphors.Besides, another broad peak appears between 40～70 ppm, and it was attributed to the second-order quadrupolar pattern of tetrahedron (four-coordinate) AlO4species caused by the lower symmetry of the crystallographic site with respect to the octahedral (six-coordinate) AlO6units.

3. 3 Morphology and microstructure

The SEM and TEM images were executed for observing the morphology and mean grain size () of YAG:Ce,Eu sample as shown in Fig. 4(a, b). It is seen thatthevalue is ~65 nm. Moreover, the detailed microstructure and composition of the YAG:Ce,Eu sample sintered at 930 ℃ were characterized by HRTEM and EDS. Fig. 4(c) shows the HRTEM image of the YAG:Ce,Eu sample and the interplanar spacing of the lattice fringes211is estimated to be 0.495 nm that is larger than the standard value of YAG (211= 0.491 nm, JCPDS: 33-0040)[33]. The EDS spectrum in Fig. 4(d) further confirms the presence of yttrium (Y), cerium (Ce), europium (Eu), aluminum (Al) and oxygen (O) elements in the YAG:Ce,Eu sample. Except for the C and Cu peaks resulting from the copper mesh, no other impurity can be detected in the sample.

Fig. 4. (a) SEM, (b) TEM, (c) HTEM and (d) EDS of YAG:Ce,Eu nanophosphors sintered at 930 ℃

3. 4 Photoluminescence properties

PL emission spectra were employed for studying the PL properties of the Ce3+- and Eu3+-codoped YAG nanophosphors. The PL spectra of YAG, YAG:Ce and YAG:Ce,Eu nanophosphors calcined at 930 ℃ under 325 nm excitation are presented in Fig. 5(a), and all characteristic emissions of Ce3+and Eu3+ions can be found. These peaks of YAG:Ce,Eu nanophosphors are assigned to the well-known transitions from the initial state50to the final states7J(= 1～4) of Eu3+ion besides the Ce3+5→ 4transitions[13,34]. In cubic YAG phase, each Y3+ion was coordinated with eight oxygen ions with2point symmetry, and the Eu3+ions replaced partial Y3+ions and exhibited similar2point symmetry[35].

Many studies on the PL characteristics of Eu3+-doped YAG nanophosphors reported that the transition of Eu3+50→71showed higher intensity than that of Eu3+50→72[35]. And the PL intensity of Eu3+50→72transition relied more strongly on the local symmetry of the Eu3+ion in host lattice than that of Eu3+50→71transition[36]. However, due to the higher degree of disorder near the surface, Eu3+ion can present lower symmetry in YAG nano- structure[37], resulting in a dominant Eu3+50→72transition intensity observed in the PL spectrum of YAG:Ce,Eu nanophosphors under 325 nm excitation. The emission at ~590 nm is due to the Eu3+50→71magnetic dipole transitions, which is insensitive to the site symmetry. The emission centering at ~609 nm is due to the Eu3+50→72electric dipole transition, which indicates that Eu3+ions occupy the non-inversion symmetric sites with low symmetry in the host lattice[38]. While the peak at ~631 nm corresponds to Eu3+50→73transition, and ~710 nm peak is due to the50→74transition of Eu3+ion, respectively[37]. The50→74transition dominates the other transitions, which should origin- nate from the formation of complicated optical cen- ters including the molecular groups of Eu3+, Ce3+and O2-ions withnvsymmetry in YAG structure[13,39,40]. The abovementioned analyses imply that the fine crystal structure of Eu3+coactivator shows a small distortion of the centrosymmetric2hpoint sym- metry, which is corresponding with the previous XRD and MAS NMR experimental results.

Fig. 5. PL (ex= 325 nm) (a), PLE (em= 532 nm) (b), PL (ex= 454 nm) (c) spectra of pure YAG, YAG:Ce andYAG:Ce,Eu samples sintered at 930 ℃

In order to examine the complicated energy transfer in YAG:Ce,Eu nanophosphor, the PLE (em= 532 nm) and PL emission (ex= 454 nm) spectra of the pure YAG, YAG:Ce and YAG:Ce,Eu samples sintered at 930 ℃were further measured and presented in Fig. 5(b) and (c), respectively. It is clear that the PLE spectra of both YAG:Ce and YAG:Ce,Eu display two excitation bands, deriving from the 4→ 51,2electron transitions of Ce3+ions. A typical broad emission band centering at ~532 nm was observed from the YAG:Ce,Eu nanophosphors, which was assigned to the typical 51→ 4(25/2) and 51→ 4(27/2) transitions of Ce3+ion[3], and the energy difference between the two energy levels was ~1500 cm-1caused by the spin-orbital coupling in crystal field. It is known that the Ce3+51→ 4(25/2,27/2) state is considerably influenced by the crystal field surrounding Ce3+ions[3]. The above solid-state quantitative27Al MAS NMR results have proved that Eu3+ions enter the dodecahedral Y3+lattice sites, which agrees with the previous literatures[31,32]. We assume that the replacement of partial dodecahedral Y3+by Eu3+ions changes the local crystal field surrounding Ce3+ions, which can influence the PL emission of Ce3+activators in YAG:Ce,Eu nano- phosphors. Furthermore, when the distance between rare ion and ligand becomes shorter, nephelauxetic effect increases, and then the emission spectra show red shift[41]. The bond length of Eu–O is shorter than that of Ce–O, thus the PL emission peak (ex= 454 nm) of YAG:Ce,Eu nanophosphors shifts to longer wavelength in comparison with that of the YAG:Ce nanophosphors (centering at 525 nm) as Eu3+ions are incorporated into YAG:Ce nanophosphors, whereas the PL intensity dramatically decreased as Eu3+ions incorporated into YAG:Ce nanocrystals. Since no impurity phase generates as Eu3+ions is incorporated into YAG:Ce nanophosphors at the calcination temperature of 930 ℃as evidenced by the XRD anlysis, the decrease of the emission intensity of Ce3+ions may be attributed to the high-efficient energy transfer from Ce3+to Eu3+ions[42], which is further evidenced by the weaker and weaker emission intensity in the PL spectra of YAG:Ce,Eu nanophosphors with increasing the Eu3+-doped concentration, as shown in Fig. 6.

Fig. 6. PL spectra (ex= 325 nm) of YAG:Ce0.0055,Eu(= 0.0011, 0.0055, 0.0550) nanophosphors with different Eu3+-doped concentration sintered at 930 ℃

4 CONCLUSION

Ce3+- and Eu3+-codopedYAG nanophosphors were prepared at low calcination temperature of 930 ℃via sol-gel method. The geometric distortion of local structure introduced by incorporated Eu3+ions could change the crystal field surrounding Ce3+in YAG:Ce,Eu nanophosphors, which played a significant role in tuning the PL emission of Ce3+activator.The PL spectrum of the Ce3+- and Eu3+-codoped YAG nanophosphors under near UV excitation consists of a classic broad 51→ 4(25/2,27/2) emission of Ce3+activator and several sharp and narrow50→7J(J = 1～4) emissions of Eu3+coactivator.The PL emission of YAG:Ce,Eu nanophosphors shifts to longer wavelength than that of YAG:Ce nanophosphors under blue excitation accompanied with Eu3+ions incorporation. Hence, YAG:Ce,Eu is a crucial PL material endowed with high-efficiency nontoxic yellow and red emissions andis extremely potential as an excellent red nanophosphor applied in optical applications.

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7 August 2017;

6 November 2017

①Financial support from the NNSF of China (No. 51171007 and 51271009)

Li Jian-Rong, male, associate professor, research field: metal chalcogenide. Fax: +86-591-63173146, E-mail: jrli@fjirsm.ac.cn; Yu Rong-Hai, male, professor, research field: magnetic material. Fax: +86-010-82317101, E-mail: rhyu@buaa.edu.cn

10.14102/j.cnki.0254-5861.2011-1802