Synthesis and Luminescent Properties of SrZrO₃:Tb³⁺ Phosphors

 

Sheetal¹, Sonika¹*, Poonam Nandal¹, S.P. Khatkar¹, Ritu Langyan²

¹Department of Chemistry, Maharshi Dayanand University, Rohtak - 124001, India

²Department of Chemistry, Kurukshetra University, Kurukshetra – 136119, India

*Corresponding Author E-mail: sssonicasingh31@gmail.com

SrZrO₃:Tb³⁺ phosphor with perovskite structure has been successfully synthesized by combustion method using glycine as a fuel. The structure, morphology and luminescent properties of material have been characterized by X-ray diffraction, scanning electron microscopy and fluorescence spectrometry. The XRD results indicate that crystals of SrZrO₃:Tb³⁺ belongs to tetragonal perovskite system. The phosphor can be effectively excited by UV light and the emission spectra results indicate green luminescence of SrZrO₃:Eu³⁺. The emission spectra comprises of stronger peaks due to transition ⁵D₄→⁷F₅ (544 nm) for the SrZrO₃:Tb³⁺ phosphor.

 

 

 

1. INTRODUCTION:

SrZrO₃ has high refractive index, wide energy gap, high melting temperature, high chemical stability, good thermal shock resistance, a relatively-low thermal expansion co efficient, low optical loss and excellent optical properties [1]. These are attractive for applications in electroceramic industry and electrochemical devices [2]. A little work has been done in the area of luminescent properties of SrZrO₃ doped with rare earths in the form of powders and single crystals [3-6]. Rare earth ions have been considered the most important optical activators for luminescent devices. The aim is to develop luminescent materials of three basic colors with higher radiative efficiency. Tb³⁺ ions are promising species that provide optical devices in blue and green color regions and various compounds have been investigated for display applications [7-10]. Tb³⁺ causes green emission at nearly 545 nm from ⁵D₄–⁷F₅ transition when doped in different hosts. In the present work, Sr_1-xZrO₃:xTb³⁺ phosphors is synthesized by solution combustion method using glycine as an organic fuel at 550°C temperature.

 

The combustion synthesis provides an interesting alternative over other elaborated techniques because it offers several attractive advantages such as: simplicity of experimental set-up; surprisingly short time between the preparation of reactants and the availability of the final product; and being cheap due to energy saving.

 

2. MATERIAL AND METHODS:

2.1. Powder Synthesis:

The starting reagents are high purity Sr(NO₃)₂.2H₂O, ZrN₂O₇, Tb(NO₃)₃.6H₂O and glycine. Sr_1-xZrO₃:xTb³⁺ is synthesized by combustion method. According to nominal composition of Sr_1-xZrO₃:xTb³⁺ (x=0.01, 0.02, 0.03 and 0.04), a stoichiometric amount of metal nitrates were dissolved in minimum quantity of deionized water in 200 mL capacity pyrex beaker. Then glycine was added in this solution with molar ratio of glycine to oxidizer based on total oxidizing and reducing valencies of oxidizer and fuel according to concept used in propellant chemistry. Finally the beaker containing solution was placed into a preheated furnace at 550°C. The material undergoes rapid dehydration and foaming followed by decomposition, generating combustible gases. These volatile combustible gases ignite and burn with a flame yielding voluminous solid. Glycine was oxidized by nitrate ions and served as a fuel for propellant reaction. The powders obtained were again calcined at 700°C, 900°C, 1100°C and 1350°C temperatures for 3 h to increase the brightness and crystallinity of as-prepared phosphors.

 

2.2. Powder Characterization Techniques:

Crystal phase of SrZrO₃:Tb³⁺ powders were characterized by Rigaku Ultima-IV X-ray powder diffractometer with CuKα radiation to record the patterns in 2θ range of 20°-65°. Surface morphology was evaluated using Jeol JSM-6510 scanning electron microscope. Excitation and emission spectra of powders in the ultraviolet-visible region were obtained using Hitachi F-7000 fluorescence spectrophotometer with Xe- lamp as the excitation source. All the properties were investigated at room temperature.

 

3. RESULTS AND DISCUSSION:

3.1. X-ray Diffraction Studies:

The X-ray diffraction patterns of Sr_0.97Tb_0.03ZrO₃ powders as prepared and calcined at 700°C, 900°C, 1100°C and 1350°C temperatures are shown in fig. 1. When introduced in SrZrO₃ host, the dopant Eu³⁺ (0.095 nm) does not have a single choice between substitution sites of Sr²⁺ ion (0.118 nm) and Zr⁴⁺ (0.072 nm). Due to similar size difference of Eu³⁺ with Sr²⁺ and Zr⁴⁺, Eu³⁺ replaces both the ions partially in the host lattice. The XRD patterns of as-prepared Sr_0.97Tb_0.03ZrO₃ powder consists many additional peaks related to undecomposed Sr(NO₃)₂ phase (JCPDS no. 04-0310) in addition to formation of SrZrO₃ phase. At lower calcination temperatures the impurities due to ZrO₂ (JCPDS no. 49-1746) and SrCO₃ (JCPDS no. 84-1778), exists along with SrZrO₃ phase [11]. But the additional phases of ZrO₂ and SrCO₃ start to diminish with the rise in temperature. Hence, the powders were calcined at higher temperature in order to obtain single phase. At 1350°C, all the diffraction peaks can be indexed to tetragonal perovskite SrZrO₃ phase (JCPDS no. 48-1049), the main peaks near 30.12°, 30.87°, 44.16° and 54.81° can be assigned to (114), (200), (215) and (314) planes  respectively. No peaks from other phases can be detected at 1350°C temperature indicating complete phase formation of the SrZrO₃:Tb³⁺ phosphors. The effect of Tb³⁺ ions doping on SrZrO₃ lattice seems to be negligible as XRD patterns remains the same at such low dopant concentration level.

 

Fig.1. XRD patterns of Sr_0.97Tb_0.03ZrO₃ powder calcined at various temperatures along with standard data of SrZrO₃ (JCPDS no. 48-1049).

 

The size of the crystallites can be estimated using Scherrer equation, D=0.94λ/βcosθ, where D is average crystallite size, λ is X-ray wavelength (0.15418 nm), and θ and β are diffraction angles and full-width at half-maximum (FWHM, in radian) of an observed peak, respectively. The average crystallite size has been determined from the peaks corresponding to 2θ=30.87° due to (200) plane of SrZrO₃ tetragonal lattice. The average size of the crystallites is found to be 28 nm for the material calcined at 900°C, 33 nm for the material calcined at 1100°C and 52 nm for the material calcined at 1350°C temperatures respectively. It can be observed from the calculated results that with the increase of calcination temperature crystallite size also increases.

 

3.2. Morphological Characteristics:

The scanning electron microscope images of Sr_0.97Tb_0.03ZrO₃ as prepared and calcined at various temperatures 900°C, 1100°C and 1350°C are represented in fig. 2(a-d) respectively.

 

Fig.2. SEM micrographs of Sr_0.97Tb_0.03ZrO₃ (a) as-synthesized, calcined at (b) 900°C (c) 1100°C and (d) 1350°C.

 

For the as-prepared products shown in fig. 2(a) agglomerates of varying shape and sizes can be noticed. For Sr_0.97Tb_0.03ZrO₃ sample calcined at 900°C as shown by fig. 2(b) tetragonal morphology of the particles could be noticed which is highly agglomerated. With further rise of the calcination temperature up to 1100°C clear formation of tetragonal shape particles could be observed. Hence a uniform distribution of particles can be observed having small size distribution from fig. 2(c). The size of tetragonal particles continues to grow with increasing temperature and agglomeration phenomenon continues to decrease. Finally at 1350°C uniformally distributed non agglomerated particles are obtained shown in fig. 2(d). It can be concluded that the particle size increased and agglomeration decreased with the increase of temperature.

 

3.3. Luminescent Properties:

Fig. 3 shows the excitation spectrum of Sr_0.97Tb_0.03ZrO₃ calcined at 1350°C at an emission wavelength of 544 nm. The excitation spectrum consists of a broad excitation band in the range from 200 nm to 240 nm with a maximum at about 222 nm and a series of sharp peaks between 300 nm and 400 nm with very low intensity in comparison to broad band. The high intensity band at 222 nm is assigned to spin-allowed 4f⁸–4f⁷5d¹ transitions of Tb³⁺ ions in the SrZrO₃ host lattice. The peaks from 300 nm to 390 nm are assigned to Tb³⁺ intra-4f (4f⁸–4f⁸) transitions from the ground ⁷F₆ state to higher energy levels.

 

The emission spectra of SrZrO₃:Tb³⁺ under excitation wavelength of 222 nm are shown in fig. 4 (a,b). The emission spectra of SrZrO₃:Tb³⁺ comprises of stronger peaks due to Tb³⁺ transitions; ⁵D₄→⁷F₅, ⁵D₄→⁷F₄ and ⁵D₄→⁷F₃ at 544 nm, 586 nm and 622 nm wavelengths respectively [12,13]. The green emission peak at 544 nm dominates due to transition ⁵D₄→⁷F₅ (544 nm) for the SrZrO₃:Tb³⁺ phosphor. The relative photoluminescence intensity of Sr_0.97Tb_0.03ZrO₃ as a function of temperature, at λ_ex=243 nm is shown in fig. 4(a). The photoluminescence intensity of the phosphors is found to increase with increase in calcination temperature reaching a maximum value at 1350°C. These observations can be explained by the fact that pure phase cannot be attained at a temperature lower than 1350°C. Fig. 4(b) displays the dependence of PL intensity of SrZrO₃:Tb³⁺ on the concentration of dopant ion Tb³⁺. It is quite clear that PL intensity of Sr_1-xZrO₃:xTb³⁺ (x=0.01-0.04) increases with increasing Tb³⁺ concentration, reaching a maximum value at x=0.03 and thereafter decreases with further increase in Tb³⁺ concentration because of mutual Tb³⁺-Tb³⁺ interactions.

 

Fig.3. Excitation spectrum of Sr_0.97Tb_0.03ZrO₃ (λ_em=544 nm) sample calcined at 1350°C.

 

Fig.4. (a) Emission spectra of Sr_0.97Tb_0.03ZrO₃ (λ_ex=222 nm) showing variation of emission intensity as a function of temperature and (b) relative PL intensity of Sr_1-xZrO₃:xTb³⁺ as a function of Tb³⁺ concentration.

 

4. CONCLUSION:

SrZrO₃:Tb³⁺ was synthesized at 550°C temperature by combustion technique employing glycine as an organic fuel. The solid obtained was again calcined at 700°C to 1350°C for 3 h to increase the brightness. XRD results indicate that the particles calcined at 1350°C temperature crystallize in pure tetragonal perovskite SrZrO₃ phase (JCPDS no. 48-1049). At 1350°C uniformally distributed non agglomerated tetragonal shape particles are obtained as shown by SEM image. The phosphor have grain size less than 1 µm. Optimum concentration of Eu³⁺ was found to be 3 mol% of Eu³⁺ in the host lattices. The main emission peak at 544 nm has been assigned to the magnetic dipole transition ⁵D₄→⁷F₅ under λ_ex=222 nm.

 

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Received on 01.12.2017         Modified on 05.01.2018

Accepted on 23.01.2018         © AJRC All right reserved