Research of Materials Science September 2014, Volume 3, Issue 3, PP.52-56
Synthesis and Luminescence Properties of New Red Phosphor BaZnBO3F:Eu3+ for Light-Emitting Diode Wanping Chen#, Xiaoyan Dai, Xin Yang, Yan Liu Huaihua Key Laboratory of Functional Inorganic & Polymeric Materials, Department of Chemistry and Chemical Engineering, Huaihua University, HuaiHua 418008, P. R. China #
Email: cwp0918@aliyun.com
Abstract A series of BaZnBO3F:xEu3+ (x = 0.005, 0.01, 0.02, 0.04, and 0.06) phosphors were synthesized by a high temperature solid-state method. X-ray diffraction patterns, excitation and emission spectra were recorded to investigate the phosphors. The phosphor showed two strong absorption bands at ~395 nm and ~466 nm, which just overlap the emission bands of UV and blue LED chip. Simultaneously the phosphor show a predominant emission of 5D0 − 7F2 at ~610 nm. The optimal doping concentration is about x=0.04. The results show that BaZnBO3F:xEu3+ have potential application in white LED. The crystal structure and the site occupancy of Eu3+ were investigated simply according to the Rietveld refinement results. Keywords: Luminescence; Phosphor; BaZnBO3F
1 INTRODUCTION White light-emitting diode (LED) have been considered as the fourth generation of lighting source, which can be easily fabricated by combining a blue or UV LED chip with some phosphors [1, 2]. For example, the commercially available white LED is the combination of a blue LED chip (460 nm) with a yellow phosphor (Y,Gd)3Al5O12:Ce3+. For the two white LED, a suitable red phosphor is indispensable to obtain a high quality white-light with a high color rendering index (CRI) and a low colored temperature [3]. In general, red phosphors can be achieved with some inorganic compounds activated by Eu3+ [4-8], Eu2+ [9,10], Mn4+ [11,12], Sm3+ [13,14], or Pr3+ [15,16]. As a potential LED red phosphor, one vital prerequisite is that the phosphor must be effectively excited by the emission of near-UV (350 − 410 nm) or blue LED (420 − 480 nm) chips. Therefore, most existing red phosphors for fluorescence lamps or color television cannot be used as LED phosphors. Currently, only several sulfide and nitride-based phosphors activated with Eu2+ such as SrS:Eu2+ and Sr2Si5N8:Eu2+ are commercially available for white LED. However, some drawbacks limit their extensively application in white LED. For examples, SrS:Eu2+ is sensitive to water and Sr2Si5N8:Eu2+ require rigorous synthesis condition. As an alternative, some phosphors activated by Eu3+ may be potential application in white LED because of their strong and linear emissions at 610-615 nm [2], if the absorptions bands of Eu3+ at ~395 nm (7F0 − 5L6) and ~465 nm (7F0 − 5D2) can be effectively enhanced. Therefore, much attention has been attracted to investigate new red phosphors activated by Eu3+ for white LED [4-8]. As host material, the complex oxyhalide, such as haloaluminate, haloborate, halosilicate, and halophosphate, have attracted much attention because of their abundant structure types, adjustable cation site environments, and promising optical properties [17-20]. Recently, a new compound BaZnBO3F was reported to possess some excellent optical properties, which built up by five coordinated trigonal bipyramidal ZnO3F2 polyhedra and triangular BO3 groups [21]. In the present work, BaZnBO3F was used as host material to explore LED red phosphor activated by Eu3+. In BaZnBO3F, Eu3+ shows strong absorptions at ~395 and ~466 nm and dominant 5D0 − 7F2 (~610 nm) emission. The results indicate that the phosphor have potential application in white LED. - 52 http://www.ivypub.org/rms
2 EXPERIMENTAL All the phosphors with nominal composition Ba1-xZnBO3F:xEu3+ (x = 0.005, 0.01, 0.02, 0.04, and 0.06) were synthesized by a high temperature solid-state method. Appropriate amounts of starting materials containing BaCO3 (analytical reagent, AR), BaF2 (self preparation), ZnO (AR), H3BO3 (AR), and Eu2O3 (99.99%) were thoroughly mixed in an agate mortar and precalcined at 700 oC for 2 h, and then reground and calcined at 800 oC for 1 h under a ambient atmosphere. The X-ray diffraction (XRD) patterns of all the phosphors were recorded with a X-ray powder diffractometer (Ultima IV, Rigaku, Japan) with Cu Kα radiation (λ = 1.5045 Å). Simultaneously, to perform a Rietveld refinement, high quality XRD data was collected in the 2θ range 18 − 120 ºwith a step of 0.02 ºand accumulation time of 2 second per step. The Rietveld refinement was performed with the General Structure Analysis System (GSAS) software [22] using EXPGUI graphical interface [23]. The emission and excitation spectra were recorded with a Shimadzu RF-5301PC fluorescence spectrophotometer. All measures were carried out at a room temperature.
3 RESULTS AND DISCUSSION Fig. 1 presents the XRD patterns of BaZnBO3F:xEu3+ (x = 0.005, 0.01, 0.02, 0.04, and 0.06). All XRD patterns well agree with the calculated XRD pattern of BaZnBO3F according to its crystal structure data [21], and no impurity phase exists at first glance. This indicates that the Eu3+ ions completely entered the host crystal. In fact, minor impurities of BaF2 and ZnO were detected in the phosphors [21], especially for the phosphors with high doping level. In the XRD patterns recorded for Rietveld refinement, the impurity phases can be observed obviously. Considering the mismatch of charge and size between Ba2+ and Eu3+ and the existence of impurities, the doping concentration (x value) is limited to the range 0.005 − 0.06.
FIG. 1 XRD PATTERNS OF BAZNBO3F:XEU3+ (X = 0.005, 0.01, 0.02, 0.04, AND 0.06).
In order to examine the influence of Eu3+ on the crystal structure of BaZnBO3F, the XRD data of BaZnBO3F:0.04Eu3+ were utilized to perform the Rietveld refinement. The starting structural model was built with the crystallographic data [21]. The minor impurities BaF2 and ZnO were also refined. Fig. 2 presents the Rietveld refinement profile. The reliability factors of the refinement are Rp = 7.43%, Rwp = 5.67%, and χ2 = 2.51, indicating a good fit to the experimental data. The refined structural parameters are presented in Table 1. The compound BaZnBO3F possesses a hexagonal crystal structure with the P-6 space group and the cell unit parameters a = b = 5.0657(1) Å and c = 4.2800(1) Å [21]. In BaZnBO3F, Ba2+ coordinates with six oxygen atoms and three fluorine atoms to form a tricapped trigonal prism. The crystal structure of BaZnBO3F and the corresponding Ba2+ coordinated polyhedron are presented in the inset of Fig. 2. Because the ionic radius of Ba2+, Eu3+, and Zn2+ is 1.35, 0.974, and 0.74 Ǻ, respectively [24], one speculated that Eu3+ should preferentially substitute for Ba2+ and not for Zn2+ when Eu3+ is doped in BaZnBO3F, and the cell parameters will decrease. However, the refined results show that the cell parameters a and c are 5.086443(8) and 4.29969(4) Å, which are larger than that of the host. This is distinct from the speculation. One of possible reasons is that Eu3+ does enter Ba2+ sites and disorder the crystal structure due to the mismatch of charger, which will effectively enlarge the crystal cell parameters. - 53 http://www.ivypub.org/rms
TABLE 1 RIETVELD REFINEMENT OF BAZNBO3F:0.04EU3+ AND THE COMPARISON WITH HOST BAZNBO3F [21] Structure Space group V (Å3) Z a (Å) c (Å) χ2 Rwp (%) Rp (%)
BaZnBO3F:0.04Eu3+ hexagonal P-6 96.3378(11) 1 5.086443(8) 4.29969(4) 2.51 7.43 5.67
BaZnBO3F hexagonal P-6 95.119 1 5.0657(1) 4.2800(1)
FIG. 2 OBSERVED (CROSS), CALCULATED (RED SOLID LINE), AND DIFFERENCE (BOTTOM) RESULTS OF XRD REFINEMENT OF BAZNBO3F:0.04EU3+; THE INSET IS THE CRYSTAL STRUCTURE OF BAZNBO3F AND BA2+ COORDINATION POLYHEDRON.
To investigate the luminescence properties of Eu3+ in BaZnBO3F, the excitation and emission spectra of BaZnBO3F:0.04Eu3+ was recorded and presented in Fig. 3. By monitoring the emission of 610 nm, three strong absorption bands are observed in the excitation spectrum lettered with a in Fig. 3. The absorption band at ~250 nm is assigned to the charge-transfer band (CTB) of O2− − Eu3+, and the other two absorption bands at ~395 and ~466 nm are assigned to the f-f transition of 7F0 − 5L6 and 7F0 − 5D2 of Eu3+, respectively. Under the excitation of 466 nm, the emission spectrum contains all 5D0 − 7FJ (J = 0, 1, 2, 3, and 4) emissions (Fig.3b). Among these emissions, the 5D0 − 7 F2 emission at ~610 nm is predominant, the 5D0 − 7F0,1 emissions are weaker, and the 5D0 − 7F3,4 emissions are virtually negligible. In general, the luminescence color of Eu3+ in a specific host primarily depends on the relative emission intensity of 5D0 − 7F2 and 5D0 − 7F1. Here, the intensity ratio of 5D0 − 7F2 to 5D0 − 7F1 is about 2.0, and the chromatic coordinate is (0.627, 0.373) calculated in terms of the emission spectrum in the range of 550 − 730 nm. Under the excitation of 395 nm, a similar emission spectrum is obtained, but the emission intensity slightly decreases, as displayed by the emission spectra c in Fig. 3. These indicate that the phosphor can be excited effectively by ~395 and 466 nm. Because the excitation bands partially overlap the emission bands of UV and blue LED chips, the phosphor may be used as red LED phosphor if its luminescence properties can be improved.
FIG. 3 EXCITATION AND EMISSION SPECTRA OF BAZNBO3F:0.04EU3+. - 54 http://www.ivypub.org/rms
In addition, in order to study the site occupancy of Eu3+, the phosphor BaZnBO3F:0.04Eu3+ was excited with different wavelength light. However, under the excitation of 230, 240, 250, 260, 270, 395, 543, and 466 nm, all the recorded emission spectra are almost completely identical in their spectral profile and the intensity ratio of 5D0 − 7F2 to 5D0 − 7F1 emission. This indicates that all Eu3+ ions have similar lattice site environment, and they primarily occupy the Ba2+ sites according to the Rietveld refinement results.
FIG. 4 RELATIVE EMISSION INTENSITY OF PHOSPHORS BAZNBO3F:XEU3+ (X = 0.005, 0.01, 0.02, 0.04, AND 0.06) UNDER 466 NM EXCITATION; THE INSET IS THE COMPARISON OF INTEGRATED INTENSITY.
To obtain the optimal doping concentration, the emission spectra of a series of phosphors BaZnBO3F:xEu3+ with x = 0.005, 0.01, 0.02, 0.04, and 0.06 were recorded and presented in Fig. 4, and the comparison of their integrated intensity in the spectral range 500 − 730 nm was also presented in the inset. Under the excitation of 466 nm, all the samples show similar emission spectra with the sample BaZnBO3F:0.04Eu3+, but their emission intensities are obviously different. With the increasing doping concentration, the emission intensity gradually rises to a maximum and then decrease. The strongest emission is obtained when the doping concentration (x value) is 0.04. This indicates that the optimal doping concentration is about 0.04.
4 CONCLUSIONS Red phosphors BaZnBO3F:xEu3+ (x = 0.005, 0.01, 0.02, 0.04, and 0.06) were synthesized by a high temperature solid-state. Under the excitation of 395 and 466 nm, the phosphor show a predominant 5D0 − 7F2 (~610 nm) emission, and the corresponding chromatic coordinates is (0.627, 0.373). The optimal doping concentration of Eu3+ is about 0.04. The Rietveld refinement indicates that Eu3+ primarily occupy the Ba2+ sites. The results show that BaZnBO3F:Eu3+ may be a potential red phosphor for white LED.
ACKNOWLEDGMENT The work is funded by the National Natural Science Foundation of China (51102106).
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