Elsevier

Journal of Luminescence

Volume 169, Part A, January 2016, Pages 106-114
Journal of Luminescence

An efficient gel-combustion synthesis of visible light emitting barium zirconate perovskite nanoceramics: Probing the photoluminescence of Sm3+ and Eu3+ doped BaZrO3

https://doi.org/10.1016/j.jlumin.2015.08.032Get rights and content

Abstract

Barium zirconate (BZO) nanoceramics were synthesized using self-assisted gel-combustion route. BZO powders were characterized systematically using X-ray diffraction (XRD), transmission electron microscopy (TEM), electron paramagnetic resonance (EPR) and photoluminescence (PL). Doping of Eu3+ and Sm3+ was confirmed by XRD. Intense blue and green emission was observed in undoped sample without activator ion. PL and EPR study shows the presence of oxygen vacancy (shallow and deep defect) is responsible for such visible emission. Bases on PL spectroscopy; it was inferred that majority of Sm3+ ion stabilizes at Ba2+ site without inversion symmetry though oxygen vacancies are introduced in vicinity to ensure local charge compensation whereas majority of Eu3+ ion occupies Zr4+ site. The fact that in Ba1−xEuxZrO3; electric dipole transition (EDT) is much more intense than magnetic dipole transition (MDT); asymmetry ratio is found to be much greater than unity (I⪢1) indicating majority of Eu3+ at site without inversion symmetry in BaZrO3 perovskite. The actual site symmetry for Eu3+ ion in barium zirconate was also evaluated based on stark splitting pattern which comes out to be C6 in BaZrO3.This is also reflected in the trend of Judd Ofelt (JO) parameter; Ω2 value was found to be greater than Ω4 indicating high covalency and low symmetry around Eu3+ ion.

Introduction

Perovskite oxides with ABO3 formula are special class of compounds which found application in all areas of science and technology. This includes cheap solar cell [1], multiferroic material [2], energy harvesting device [3], photocatalysis [4], anode material in solid oxide fuel cell [5] etc. They can be tuned to different properties based on doping site i.e. A or B-site doping. Barium Zirconate (BZO) in particular has been explored for various technological applications because of its interesting physico-chemical properties like wide band gap, high refractive index, high melting temperature, small coefficient of thermal expansion, doesn’t undergo any phase change upto 1600 K and high dielectric permittivity [6], [7], [8]. These properties make BZO an interesting material that can be used for various applications like proton-conducting solid oxide electrolysis [9], photocatalyst [10], [11], electromechanical material [12], luminescence host [13] etc. It is also used as corrosion protective agent in growth process of superconductor [14]. Because of its excellent thermo-mechanical properties it is also used in aerospace industry [15].

Luminescent materials have fetched significant interest due to their wide range of technological applications in various fields such as biomedical [16], [17], LEDs [18], mid IR laser [19], solar energy [20] etc.

Recently lots of research interests are focused on luminescence studies pertaining to ABO3 type distorted perovskite oxide like SrTiO3 [21], PbZrO3 [22], PbTiO3 [23], CaTiO3 [24], MgTiO3 [25], SrZrO3 [26], [27], [28], [29] etc. where local defects in the perovskite structure is mostly responsible for photoluminescence in visible region. The local defects in such cases are mostly oxygen vacancies (OV); which can be neutral (VO), singly ionized (VO) or doubly ionized (VO) depending upon the thermal treatment and synthesis condition and methodology. The advantage of such perovskite based materials is their ability to stand out from the other as far as luminescence study is concerned because they are very robust host and are sufficiently conductive to release the accumulated charges on the surface of phosphor particles which makes them suitable candidate for field emission display (FED) and plasma display panel (PDP) devices [30]. They are also reported to be highly resistant to high-density electron irradiation because of which thermal stability and luminescence efficiency is maintained even with prolonged columbic loading [31].

Barium zirconate is one such perovskite which is an excellent luminescence host and used in various rare earth doped phosphor such as BZO: Eu [6], [32], BZO: Eu, Ti [33], and BZO: Yb, Tm [34] etc. Undoped BZO is reported to give visible light emission in blue–green region which is attributed to presence of oxygen vacancy or other structural defect produced during the synthesis [7], [35], [36], [37], [38], [39].

As far as literature is concerned there is no report on luminesce of Sm3+ in BZO and there is only report by Hano-Gonzalez et al. which explains the site occupancy of Eu3+ in BaZrO3 [40]. We have used time resolved photoluminescence spectroscopy (TRPLS) as a tool to probe the local site of Eu3+ and Sm3+ in BZO. Understating the local site of lanthanide ion in doped host where multiple sites are available for occupancy is very important to have better understanding of structure–property correlation to optimize its performance.

Such information can’t be obtained from XRD at low doping concentration; particularly for phosphor material where level of doping is very small otherwise the problem of concentration quenching will comes into picture which will affect the luminesce intensity.

As far as structure is concerned BZO has cubic structure with 12-coordination for Ba2+ and 6- coordination for Zr4+. We wanted to the probe the site occupancy of Eu3+ and Sm3+ through time resolved photoluminescence spectroscopy.

Eu3+ is often used as a structural probe [41], [42], [43], [44], [45], [46], [47], because of the (a) relative simplicity of its energy-level structure and the fact that it possesses non-degenerate ground (7F0) and excited (5D0) states and (b) absorption and emission spectra of this ion show marked dependence on its site symmetry in the host material.The orange emission (590–600 nm) of Eu3+ due to the magnetic dipole transition (MDT) 5D07F1 is not affected much by the site symmetry, because they are parity-allowed, while the red emission (~610–630 nm) due to the electric dipole transition (EDT) of 5D07F2, being hypersensitive, is affected by the site symmetry of Eu3+ ion. In a crystal site inversion symmetry the EDT are strictly forbidden and the MDT are usually the dominant emission line. In a site without inversion symmetry the strength of EDT is higher. The 5D07F2 transition is usually the strongest emission line in this case, because transition with ΔJ=±2 are hypersensitive to small deviation from inversion symmetry. The symmetry around the lanthanide ion can thus be obtained from the shape of the emission spectrum of the Eu3+ ion.

In case of Sm3+; 4G5/26H7/2J=±1) is a partly magnetic dipole (MD) and partly electric dipole(ED) emission band whereas 4G5/26H5/2 is purely MD and 4G5/26H9/2 is purely ED natured [48].The magnetic dipole transition (MDT) 4G5/26H5/2 does not depend on chemical surroundings of the luminescent centre and its symmetry. However, the hypersensitive 4G5/26H9/2 transition is magnetic-dipole forbidden and electric-dipole allowed and its intensity increases as the environmental symmetry become lower.

In this work we have synthesized barium zirconate nanoparticle using gel-combustion route and characterised it with X-ray diffraction (XRD), transmission electron microscopy (TEM), electron paramagnetic resonance (EPR) and photoluminescence (PL) spectroscopy. We have doped 1.0 mol% of Eu3+ and Sm3+ and explore its local structure and photophysical properties using TRFS.

Section snippets

Sample preparation

Analytical grade chemicals of ZrO(NO3)2·xH2O (AR Grade, Loba Chemie), Ba(NO3)2 (AR Grade, Merck), NH4NO3 (AR Grade, Chemico Fine Chemicals) and citric acid (AR Grade, Chemico fine Chemicals C6H8O7·H2O) were used as starting reagents for the synthesis of barium zirconate perovskite nanoparticles. Initially diluted solution of barium and zirconyl nitrate was prepared in deionized water. Then solution of ammonium nitrate prepared initially was transferred to barium nitrate solution followed by

Phase identification and structure

Fig. 1 shows the X-ray diffraction (XRD) pattern of BZO hosts and BZO: 0.01Ln3+(Ln3+=Sm3+/Eu3+) samples, which can reveal their chemical compositions and phases. All the diffraction peaks can be indexed to the pure cubic phase of BaZrO3 (JCPDS 06-0399). No traces of any other impurities diffraction peaks (BaO, Eu2O3 or Sm2O3) were seen in doped sample which indicates a homogenous solid solution of BaZrO3 and Eu3+/Sm3+. Thus it can be said that doping of lanthanide ion i.e. Sm3+ or Eu3+ ions

Conclusion

BaZrO3 nanoceramics were synthesized using self-assisted gel-combustion route using citric acid as a fuel. XRD shows the formation of pure cubic BZO phase at 600 °C. PL studies on BZO sample shows the presence of strong blue and green emission band at 440 and 515 nm respectively. Based on EPR spectroscopy, such strong emission is attributed to presence of singly ionized oxygen vacancy in BZO perovskite. More so the presence of violet blue emission is attributed to shallow defect and green

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