2TiO3 ferroelectric ceramics

Materials Letters 115 (2014) 129–131

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Materials Letters journal homepage: www.elsevier.com/locate/matlet

Bright green emission in Ho doped Bi1/2Na1/2TiO3 ferroelectric ceramics T. Wei a,n, Z. Chang b, Q.J. Zhou a, D.M. An a, Z.P. Li a, F.C. Sun a a b

College of Science, Civil Aviation University of China, Tianjin 300300, PR China Department of Chemistry, Tianjin Key Lab on Metal and Molecule-based Material Chemistry, Nankai University, Tianjin 300071, PR China

art ic l e i nf o

a b s t r a c t

Article history: Received 2 April 2013 Accepted 15 October 2013 Available online 22 October 2013

A series of Ho3 þ doped Bi1/2Na1/2TiO3 (BNTO: x Ho3 þ , x ¼0.0, 0.002, 0.005, 0.01, 0.02, 0.03, 0.04, 0.05, and 0.07) ceramics were synthesized by a solid-state reaction, and their structural, photoluminescence, and ferroelectric properties were investigated. Bright green emission of BNTO: x Ho3 þ was first observed at room temperature, and what is more, the green emission can be directly excited by blue light radiation which indicates BNTO: x Ho3 þ can act as a highly efficient phosphor. Furthermore, the optimized photoluminescence is observed in BNTO: x Ho3 þ with x ¼0.01 sample which also shows good ferroelectric properties. It is believed that BNTO: x Ho3 þ with x ¼ 0.01 may act as a potentially multifunctional optical-electro material. & 2013 Elsevier B.V. All rights reserved.

Keywords: Ceramics Luminescence Ferroelectrics

1. Introduction

2. Experimental details

Demand for white light-emitting diodes (W-LEDs) as a potential candidate for replacement of conventional incandescent and fluorescent lamps is high owing to their significant power saving, higher luminous efficiency, longer lifetime, and reliability [1,2]. To obtain promising W-LEDs [3,4], it is necessary to develop novel multi-color emission phoshphors in which the strong absorption locates in the blue or N-UV spectral region and strong light emission locates in the visible range. As a lead-free ferroelectric (FE) material, Bi1/2Na1/2TiO3 (BNT) has been intensively studied for their applications in ferroelectrics and piezoelectrics [5–8]. Besides the FE properties, very recently, BNT was confirmed as one of the good candidates for the production of red phosphor for W-LEDs [9–11]. However, most researches mainly focused on the red emission in rare earth ions (Re3 þ ) doped BNT. Up to now, there are few luminescent studies except red emission. However, it is important to develop novel multi-color emission phoshphors for W-LEDs. Thus, it is a proper time to address the photoluminescence feature of BNT doped by non-red activator ion. Specifically, to explore the green emission, Ho3 þ was deliberately introduced in BNT system to substitute for Bi3 þ -site considering that Ho3 þ is an efficient green emission activator.

The Bi1/2  xNa1/2TiO3: xHo3 þ (BNTO: x Ho3 þ ) (x ¼0.0, 0.002, 0.005, 0.01, 0.02, 0.03, 0.04, 0.05, and 0.07) ceramics were synthesized by a solid-state reaction [12,13]. Phase analysis and crystal structures were characterized using X-ray diffraction (XRD) (DX-2000 diffractometer). The photoluminescence (PL) spectra at room temperature were recorded by using Jobin Yvon HR320 fluorescence spectrophotometer. Electrodes were fabricated with fired-on silver paste for electrical measurements. The FE behaviors were measured with a Radiant Precision Multiferroic Tester (Radiant Technologies Ltd., Albuquerque, NM) in a standard mode.

n

Corresponding author. Tel.: þ 86 15122848807; fax: þ 86 22 24092514. E-mail address: [email protected] (T. Wei).

0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.10.051

3. Results and discussions Fig. 1 presents the room temperature XRD patterns of BNTO: x Ho3 þ (0.0rx r0.07) ceramics. The diffraction peaks of BNTO: x Ho3 þ can be indexed according to the single BNT phase and agree well with the Joint Committee for Powder Diffraction Standards (Card no. 36-340) [11]. No other secondary phases such as Bi2O3, NaCO3, Ho2O3, and TiO2, are detected in current XRD pattern. With x increase, the diffraction peaks of shift to higher angle side as detected in Fig. 1. For example, the (202) diffraction peak shifts from 2θ¼46.701 to 2θ¼47.101 as x ranges from 0.0 to 0.07. Considering the radii of Bi3 þ (1.30 Å, 12 CN) and Ho3 þ (1.18 Å, 12 CN), the higher angle shift of (202) peak is reasonable which alludes the lattice distortion induced by Ho3 þ doping. Fig. 2(a) representatively shows the photoluminescence excitation (PLE) spectra of BNTO: x Ho3 þ (x ¼0.0, 0.01, 0.03, and 0.07) to elucidate the excitation paths of Ho3 þ ions. By monitoring the

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T. Wei et al. / Materials Letters 115 (2014) 129–131

Fig. 1. XRD patterns of BNTO: x Ho3 þ ceramics with different Ho3 þ -doped concentration. (a) x¼ 0.0, (b) x¼ 0.002, (c) x ¼0.005, (d) x¼ 0.01, (e) x ¼0.02, (f) x ¼0.03, (g) x ¼ 0.04, (h) x ¼0.05, and (i) x ¼0.07.

Fig. 3. Variation of emission (5S2-5I8) (a) and excitation (5I8-5G6) (b) intensity versus x. The inset of (a) shows the simplified energy level diagram of Ho3 þ . The inset of (b) gives the variation of emission (5S2-5I8) intensity of BNTO: x Ho3 þ (x ¼0.01) versus TS.

Fig. 2. PLE spectra monitored at 547 nm and PL spectra excited at 449 nm. (A) x ¼0.0, (B) 0.01, (C) 0.03, and (D) 0.07.

547 nm emission, strong excitation peaks for BNTO: x Ho3 þ (x a0.0) are detected as given in Fig. 2(a). The remarkably strong and sharp excitation peaks in the wavelength range of 410– 500 nm are owing to the typical f–f absorption of Ho3 þ [14–16].

The high-energy excitation peak around 418 nm is assigned to the 5 I8-5G5 transition. Furthermore, the intense broad excitation ranging from 430 nm to 500 nm should be corresponded to the transitions from the 5I8 ground state to the 5G6, 5K8, 5F2, and 5F3 excited states of Ho3 þ as shown in Fig. 2(a). No shifts of the excited peaks for all of the BNTO: x Ho3 þ samples are observed. More importantly, it should be noted that the remarkable excitation bands (430–500 nm) locate around the emission wavelength of commercial blue-LEDs (450–470 nm) which indicates BNTO: x Ho3 þ can act as a potential blue exciting phosphor [17,18]. The photoluminescence (PL) spectra excited by blue light 449 nm of BNTO: x Ho3 þ (x¼ 0.0, 0.01, 0.03, and 0.07) is given in Fig. 2(b). Under the resonant excitation, the luminescence spectra of BNTO: x Ho3 þ dominated by three emission bands peaking around 547 nm, 655 nm, and 753 nm owing to intra f–f transitions of Ho3 þ ions are obtained. To clearly illustrate the PL process, the inset of Fig. 3(a) represents the simplified energy level diagram of Ho3 þ . It is believed that these bands in PL spectra present characteristic 5S2, 5F5, 5I4-5I8 transitions. Among them, the transition at 547 nm (5S2-5I8) has the maximum intensity which

T. Wei et al. / Materials Letters 115 (2014) 129–131

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representatively shows the polarization–electric field (P–E) hysteresis loops under different external electric field (E) for x¼ 0.0 and x¼ 0.01. Well defined P–E loops can be observed in Fig. 4 [21]. Under E ¼100 Kv/cm, the remanent (Pr) and saturate (Ps) polarization is about 66 μC/cm2 and 81 μC/cm2 for x ¼0.0 ceramic, however, the values of Pr and Ps are about 47 μC/cm2 and 63 μC/cm2, respectively for x ¼0.01 sample. Owing to different chemical character between Ho3 þ and Bi3 þ , for example, Bi3 þ has one lone pair of electron, whereas Ho3 þ has no lone pair, the substitution of Ho3 þ at Bi3 þ sites can lead to the partial destruction of ferroelectric long range order in BNT. Therefore, the reduction of polarization intensity is obtained in BNTO: x Ho3 þ system. Furthermore, the inset of Fig. 4(b) gives a luminescence photograph of x ¼0.01 obtained in darkness by a common digital camera under the excitation of a blue commercial LED (3 W, 450–455 nm). Bright green light emission was clearly observed by naked eyes at room temperature.

4. Conclusion In conclusion, BNTO: x Ho3 þ ceramics were synthesized. The PL properties of BNTO: x Ho3 þ were first reported. Under the 449 nm excitation, bright green emission centered at 547 nm was observed. Optimized PL is realized for x¼0.01 sample. The FE feature was also measured for BNTO: x Ho3 þ system. The value of Pr and Ps is about 47 μC/cm2 and 63 μC/cm2, respectively for x¼0.01. The multifunctional features of BNTO: x Ho3 þ were confirmed.

Acknowledgments This work was supported by the Natural Science Foundation of China (51102277 and 51002183), NNSF of China (21202088), Fundamental Research Funds for the Central Universities (ZXH2012P008). References Fig. 4. P–E hysteresis loops of x¼ 0.0 (a) and x ¼0.01 (b) under different E. The inset of (b) presents the luminescence photograph of x¼ 0.01 obtained in darkness by a common digital camera.

corresponds to the green light emission. This is in agreement with the previously reported literature [16]. Furthermore, to optimize the Ho3 þ doping concentration, the Ho3 þ doping concentration was varied according to BNTO: x Ho3 þ (x ¼0.0, 0.002, 0.005, 0.01, 0.02, 0.03, 0.04, 0.05, and 0.07). Fig. 3 gives the variation of the emission (e.g. the 5S2-5I8 emission peak) and excitation (e.g. the 5I8-5G6 excitation peak) intensity with x. It is observed that the emission and excitation light intensity increases with an increase in x until a maximum is reached at x ¼0.01, after which the intensity falls steadily as x further increases due to the concentration quenching effect [19,20]. It is believed that the distance between these Ho3 þ ions becomes smaller with x increase and, as a result, the nonradiative pathway by energy transfer among the Ho3 þ ions becomes the preferential one. Thus, the optimum dopant concentration of Ho3 þ ions for BNTO: x Ho3 þ is 0.01. In addition, the relationship of luminescent intensity (5S2-5I8) for BNTO: x Ho3 þ with x ¼0.01 versus the sintered temperature (TS) is also presented in the inset of Fig. 3(b). The luminescent intensity increases with an increase of TS which indicates the enhanced crystallinity of BNTO: x Ho3 þ (x ¼0.01). Besides the PL properties, BNTO: x Ho3 þ can also act as important FE material. To prove the multifunctional properties, we have also carried out FE properties measurement. Fig. 4

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