Bi0.5Na0.5TiO3:Eu3+: An intense blue converting red phosphor

Author's Accepted Manuscript

Bi0.5Na0.5TiO3:Eu3 þ : An intense blue converting red phosphor Haiqin Sun, Qiwei Zhang, Xusheng Wang, Tao Zhang

www.elsevier.com/locate/matlet

PII: DOI: Reference:

S0167-577X(14)01037-4 http://dx.doi.org/10.1016/j.matlet.2014.05.212 MLBLUE17148

To appear in:

Materials Letters

Cite this article as: Haiqin Sun, Qiwei Zhang, Xusheng Wang, Tao Zhang, Bi0.5Na0.5TiO3:Eu3 þ : An intense blue converting red phosphor, Materials Letters, http://dx.doi.org/10.1016/j.matlet.2014.05.212 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Bi0.5Na0.5TiO3:Eu3+: an intense blue converting red phosphor Haiqin Sun 1, Qiwei Zhang 1*, Xusheng Wang2*, Tao Zhang1 1

School of Materials and Metallurgy, Inner Mongolia University of Science and Technology, 7 Arden street, Baotou 014010, China 2

Functional Materials Research Laboratory, Tongji University, 1239 Siping Road, Shanghai 200092, China

Abstract An efficient red-emitting phosphor, Bi0.5Na0.5TiO3:Eu3+, was synthesized by the solid-state reaction. The Bi0.5Na0.5TiO3:Eu3+ phosphor showed a strong red emission corresponding to the 5D0→7F2 (618 nm) transition of Eu3+ under blue excitation (464 nm). The emission intensity reached its maximum when Eu3+-doped concentration is 0.25 mol. Furthermore, Bi0.5Na0.5TiO3:0.25Eu3+ revealed high quantum yield (η=0.50) and smaller full width at half-maximum (FWHM) values (5nm). These results show that Bi0.5Na0.5TiO3:Eu3+ material is a promising red phosphor for application in W-LEDs with blue LEDs chip. Keywords: Ceramics, Luminescence, Ferroelectrics

1.

Introduction

White light-emitting diodes (W-LEDs) have recently attracted much attention in the field of solid-state lighting because of their low power consumption, maintenance, long lifetime and high efficiency. Currently, the commercially available W-LEDs are the combination of blue-emitting InGaN chip and yellow YAG:Ce3+ phosphor [1]. However, poor color-rendering index (Ra) of ～80 and high correlated color temperature due to a lack of red-emitting component have limited its potential applications. To

*

Corresponding author. Fax.: +86 472 5951536; +86 21 65985179 e-mail: [email protected], [email protected]

1

overcome this disadvantages, a complementary red phosphors excited by blue light or InGaN LED chip (～465nm) are necessarily adopted to compensate the red color deficiency. Up to now, only a few red phosphors are suitable for blue light LED chip excitation, and mainly based on Eu2+ or Eu3+ ions in different hosts, like Alkali earth sulfides/oxysulfides and nitrides/oxynitrides: Y2O2S:Eu3+, CaZnOS:Eu2+, and M2Si5N8:Eu2+ (M=Sr, Ca, Ba), etc. [2-4]. Unfortunately, these red phosphors have their own problems when they are used to design white LEDs, because of chemical instability, lower red emission efficiency or high cost. Therefore, it is imperative to develop a novel red emitting phosphor with good chemical stability and high efficiency in blue spectral region for W-LEDs. Sodium bismuth titanate Bi0.5Na0.5TiO3 (BNT) with a complex perovskite structure exhibits excellent piezoelectric and ferroelectric properties, which is considered as a promising candidate material for non-lead piezoelectric material. In the last several decades, most of studies are mainly focused on modifying their piezoelectric properties [5]. Recently, Pr3+ doped Bi0.5Na0.5TiO3 have been reported by our groups [6], it showed a red-emitting property at 610 nm from 1D2→3H4 transition of Pr3+, but its lower efficiency is not satisfactory for white LED applications. Compared with the emission of Pr3+, Eu3+ exhibits more efficient 5D0→7F2 emission in red wavelength region under UV to blue light excitation, which has been widely investigated for solid-state lighting and display [2,3]. In addition, Bi3+ ion can also act both a sensitizer and activator to realize the red emission [7]. Therefore, we consider that Eu3+-doped Bi0.5Na0.5TiO3 host may possess high efficient red emission for applications in white LEDs. To our knowledge, this is the first report of a study of Eu3+ activator in BNT host, an efficient red BNT:Eu3+ phosphor can be excited by blue light (464 nm). In the present study, we report our investigations on the structural and photoluminescence properties of Eu3+ doped BNT phosphors fabricated by conventional solid-state reaction method.

2

2. Experimental procedure The Eu3O2 (99.99%, Tianjin Chemical Reagent), Bi2O3(99%, Alfa Aesar), Na2CO3 (99.95%, Tianjin Chemical Reagent) and TiO2 (99.9%, Alfa Aesar) powders as starting materials were weighed according to the formula: (Bi0.5Na0.5)1-xEuxTiO3 (where 0≤x≤0.35, abbreviated as BNT:xEu3+). The details of powder preparation are referenced in Ref. (6). The samples were prepared via the conventional solid-state reaction process and sintered at 1170 oC for 4 h. X-ray diffraction (XRD, D8 Advance, Germany) was employed to identify phase structure of materials. The photoluminescence (PL) and photoluminescence excitation (PLE) spectra at room temperature were tested using a spectrofluorometer (F-4600, HITACHI, Japan). The quantum yields and decay curve of samples were measured by the fluorescence spectrometer (steady state, lifetime, phosphorescence) (FLS920, Edinburgh Instruments, Britain). 3. Results and discussion The representative XRD patterns of the BNT:xEu3+ (x=0.05 to 0.35) samples are shown in Figure 1. All samples show a rhombohedral Bi0.5Na0.5TiO3 perovskite phase (PDF#46-0001) and a second phase. The second phase can be indexed based on the standard X-ray pattern of the Eu2O3(PDF#34-0072). The peaks of phase Eu2O3 become increasingly stronger as the Eu3+ content increases from x=0.05 to 0.35. The shift of diffraction peaks is not obviously observed with the increase of Eu3+, suggesting that the concentration of Eu3+ ions preponderate over the solubility limit of Eu3+ in BNT host.

3

Fig. 1. The XRD patterns of the BNT:Eu3+ ceramics sintered at 1170 oC. Figure 2 displays the PLE and PL spectra of the BNT:0.05Eu3+ sample at room temperature. The excitation spectra of the sample were measured by monitoring the emission at 596 nm, 618 nm and 698 nm (left), respectively. The excitation spectra exhibit three sharp excitation bands centered at 464 nm, 525 nm and 590 nm. These narrow peaks are mainly ascribed to the characteristic f-f transitions of Eu3+ ions from the 7F0,1 absorption to 5D0,1,2 excited states of Eu3+ corresponding to 7F0→5D2 (464 nm), 7F0→5D1 (525 nm), 7F1→5D0 (590 nm) transitions, respectively [8]. Among three excitation peaks, the excitation peak of 7F0→5D2 (464 nm) possesses the strongest absorption in the blue region, such excitation band can be well coupled with the characteristic emission wavelength of commercial blue light-emitting diodes (LEDs). In addition, the excitation spectra monitored by different wavelength (596 nm, 618 nm, 698 nm) all exhibit the same excitation, indicating an effective energy transfer from the absorptions at the excited bands of 464 nm, 525 nm and 590 nm. The emission spectrum excited by 464 nm mainly consists of two well known 5D0 →7FJ (J=1,2) emission lines of Eu3+, located at 596 nm and 618 nm (right). Other emission peaks in 650-700 nm range from 5D0 →7FJ (J=3,4) are relatively weak. The 5D0 →7F2 transition shows the strongest red emission, which means that the Eu3+ doped BNT ceramics would emit

4

predominantly in the red spectral region at 618 nm.

Fig. 2. The PLE spectra of BNT:0.05Eu3+ monitored by different emission wavelength (λem=596 nm, λem=618 nm, λem=698 nm,) and PL spectrum excited by 464 nm. Figure 3 shows the emission spectra of BNT:xEu3+ ceramics sintered at 1170 oC with excitation at 464 nm and dependence of intensity of the 5D0 →7F2 transition on the Eu3+ concentration. The PL spectral patterns and center wavelength are not change appreciably with Eu3+ concentration. The emission intensity reaches its maximum as Eu3+-doped concentration is 0.25 mol, and further increasing Eu3+ content leads to the decrease of emission intensity due to the concentration-quenching effect [9], as shown in the inset of Figure 3. According to Judd-Ofelt theory, the strongest red emission at 618 nm (5D0 →7F2 transition) is sensitive to the local environment, belonging to the forced electric dipole (ED) transition, while the magnetic dipole transition at 596 nm (5D0 →7F1 transition) is insensitive to the local environment, therefore, with increasing Eu3+ concentration, the values of the I(5D0 →7F2)/I(5D0 →7F1)(I0-2/I0-1) gradually increase, as shown in the inset of Figure 3, suggesting that the red emission (5D0 →7F2) is more closer to the optimal color chromaticity.

5

Fig. 3. (a)The PL spectra of BNT:xEu3+ samples, the inset is the dependence of the emission intensity (5D0 →7F2) and intensity ratio I0-2/I0-1 on Eu3+ concentration. The PL decay curve of the BNT:0.25Eu3+ (5D0 →7F2 transition) is shown in Figure 4. It can be well fitted by the equation: I (t)= I0 exp(-t/τ), and the lifetime of sample is determined to be 0.59 ms. The typical PL spectrum of the BNT:0.25Eu3+ sample possesses the smaller full width of 5.0 nm at half-maximum (FWHM) value (Figure 4), such narrow band red emission is helpful to achieve high luminous output. The CIE (Commission Internationale de I'ecairage 1931) chromaticity value was calculated to be (x=0.65, y=0.34) for BNT:0.25Eu3+ samples, much closer to the National Television Standard Committee (NTSC) red (x=0.67, y=0.33). The measured quantum yield of BNT:0.25Eu3+ sample is up to η=0.50 under the excitation of 464 nm. The quantum yield value is higher than that of the commercial red emission Y2O2S:Eu3+ [2], and are almost comparable to those of yellow YAG:Ce3+ [10]. So, Eu3+-doped BNT red materials can be a promising red phosphor for the application in W-LEDs.

6

Fig. 4. The decay curve and PL spectrum of BNT:0.25Eu3+. The insets are the quantum yield and Commission International de L'Eclairage (CIE) chromaticity diagram for BNT:0.25Eu3+. 4. Conclusions In summary, Eu3+-doped BNT ceramics were fabricated by a solid-state reaction. These samples exhibited the high efficient red emission centered at 618 nm and high color rendering index. It can be efficiently excited from at 464～490 nm, which almost covers the available range of all commercial blue LEDs. Therefore, Eu3+-doped BNT materials are potential candidates of red phosphors for the application in W-LEDs. Acknowledgments This work was supported by the Natural Science Foundation of China (No. 51072136 and 50932007) and the Innovation Fund Project of Inner Mongolia University of Science and Technology (No. 2012NCL006, 2012NCL002). References [1] Shimizu Y, Sakano K, Noguchi Y, Moriguchi T. US patent no. 5998925, 7 December 1998. [2] Kottaisamy M, Jagannathan R, Rao R, Avudaithai M, Srinivasan L, Sundaram V. J Electrochem Soc 1995; 142: 3205-3209.

7

[3] Kuo TW, Liu WR, Chen TM. Opt Express 2010; 18(8): 8187-8192. [4] Zeuuner M, Hintze F, Schnick W. Chem Mater 2009; 21(2): 336-342. [5] Zhang ST, Kounga AB, Aulbach E, Ehrenberg H, Rodel J Appl Phys 2007; 91: 112906-3. [6] Sun HQ, Peng DF, Wang XS, Tang MM, Zhang QW, Yao X. J Appl Phys 2011;110: 016102-3. [7] Cao RP, Peng MY, Qiu JR. Opt. Express 2012; 20; A977-A983. [8] Chen DQ, Yu YL, Huang P, Lin H, Shan ZF, Wang YS. Acta Materialia 2010; 58:3035-3041. [9] Wang WN, Widiyastuti W, Ogi T, Wuled Lenggoro I, Okuyama K. Chem Mater 2007; 19: 1723-1730. [10] Kim JU, Kim YS, Yang H. Mater Letter 2008; 6-7:614-616. Figure captions Fig. 1. The XRD patterns of the BNT:Eu3+ ceramics sintered at 1170 oC. Fig. 2. The PLE spectra of BNT:0.05Eu3+ monitored by different emission wavelength (λem=596 nm, λem=618 nm, λem=698 nm,) and PL spectrum excited by 464 nm. Fig. 3. (a)The PL spectra of BNT:xEu3+ samples, the inset is the dependence of the emission intensity (5D0 →7F2) and intensity ratio I0-2/I0-1 on Eu3+ concentration. Fig. 4. The PL spectrum and decay curve of BNT:0.25Eu3+. The insets are the quantum yield and Commission International de L'Eclairage (CIE) chromaticity diagram for BNT:0.25Eu3+.

Research Highlights 1.

Eu3+-doped BNT red phosphors were prepared by the solid-state reaction.

2.

BNT:Eu3+ exhibits high efficient red emission (618 nm) with η=0.50.

3. BNT:Eu3+ materials are potential candidates for the application in W-LED

8