Structural and optical characterization of sol–gel derived Tm-doped BaTiO3 nanopowders and ceramics
Highlights
► Nanocrystalline Tm3+(5%)-doped BaTiO3 has been synthesized by the sol–gel method. ► The photoluminescence spectra showed typical transitions of Tm3+ ions incorporated in the BaTiO3 crystalline lattice. ► Thermoluminescence peaks were assigned to the recombination of the Tm2+-electron traps.
Introduction
The optical properties, particularly the luminescence, of nanostructured materials have attracted considerable attention [1], [2]. The luminescence of perovskite-type materials such as titanates, is observed when pure titanate samples undergo excitation above the band gap energy [3], [4]. The luminescence of these materials has been correlated to the size of the nanocrystals and the presence of oxygen vacancies in the crystalline lattice [5], [6].
Barium titanate is a well-known material and has been studied due of its electronic and electro-optic properties [7], [8], [9]. Its optical properties have attracted considerable attention and many experimental and theoretical reports have been published about the luminescence properties of BaTiO3 nanometer sized powder, bulk and thin films [1], [2], [10]. Doping with trivalent rare earth ions introduces new energy levels in the band gap and the radiation emitted from these optical transitions between these levels cover the whole optical spectrum from UV to NIR [11]. Photoluminescence (PL) properties of Eu3+ doped BaTiO3 were discussed by Strek and Alencar [11], [12] and for Ce3+ doped BaTiO3 nanocrystals by Zhang et al. [13]. Up-conversion luminescence effects have been studied on Er3+ doped BaTiO3 [14] and Tm3+-doped BaTiO3 inverted opal [15] and tri-color UC luminescence (or called UC white light) has been observed in Er3+/Tm3+/Yb3+ tri-doped dielectric BaTiO3 nanoparticles [16].
Sol–gel method has been successfully used to obtain rare-earth doped BaTiO3 nanocrystalline powders because its advantage to produce very fine primary particles and to introduce rare-earth dopant directly with the precursors [17], [18], [19], [20], [21]. Despite this there are only few reports only on Tm3+-doped BaTiO3 powder [7], [22] and are focused mainly on the electrical properties and the correlation with the doping [22].
In this work, the photoluminescence and thermoluminescence properties of the nanocrystalline Tm3+-doped BaTiO3 powders and ceramic are investigated.
Section snippets
Experimental
Ba1−xTmxTiO3, x = 0.05 or (Ba,Tm)TiO3 nanopowders were prepared by the sol–gel method. Analytic reagent grade of raw materials: barium acetate (CH3CO2)2Ba (Aldrich), titanium (IV) isopropoxide, 97% solution in 2-propanol (Aldrich), and thulium (III) oxide Tm2O3 (Aldrich), were used as starting precursors. Titanium (IV) isopropoxide, thulium (III) oxide (solved in nitric acid) and aqueous solution of barium acetate, were mixed (molar ratio [Ti]:[Tm]:Ba = 1:0.05:0.95) while stirring the mixture
X-ray diffraction
Fig. 1 shows the X-ray diffraction (XRD) spectra of (Ba,Tm)TiO3 gel precursor powder annealed at 700 °C and 900 °C for 2 h in air and, pellets sintered at 1350 °C for 1 h.
It can be seen that the powder has high degree of crystallization and all the peaks correspond to (Ba,Tm)TiO3 solid solution crystallized on the cubic BaTiO3 lattice [23]. No secondary phase is observed, suggesting that at this temperature, thulium is completely soluble in barium titanate. X-ray diffraction spectra of
Conclusions
Barium titanate (BaTiO3) doped with 5 mol% Tm3+ has been prepared by the sol–gel method. The correlation between structural and morphological characteristics of the as-obtained materials (nanopowders and ceramic) and their photoluminescence and thermoluminescence properties were investigated. The (Ba,Tm)TiO3 powders shows nanosized grains, while the ceramic shows dense and pore-free microstructure. Photoluminescence spectra have been assigned to the radiative transitions of the Tm3+ ions in the
Acknowledgment
The authors gratefully acknowledge the Romanian Research Ministry PNCDI II, Contract nr. 72-153/2008, for financial support.
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