Synthesis and photoluminescence behavior of Bi4Ti3O12 powders obtained by the complex polymerization method

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Abstract

Bismuth titanate, Bi4Ti3O12(BiT) powders were synthesized by the complex polymerization method. The structural evolution as a function of heat treatment temperature of these powders was analyzed by X-ray diffraction (XRD) and micro-Raman (MR) spectroscopy. The optical properties were verified by ultraviolet–visible (UV–vis) absorption spectroscopy and photoluminescence (PL) measurements. XRD patterns and MR spectra revealed that the BiT powders heat treated at 700 ° C for 2 h under oxygen flow crystallize in an orthorhombic structure without deleterious phases. UV–vis spectra indicated the presence of intermediary energy levels within the band gap of the powders heat treated at low temperatures. The maximum PL emissions of these materials were verified at around 598 nm, when excited by 488 nm wavelengths. Also, it was observed the presence of two broad PL bands, which were attributed to the intermediary energy levels arising from α-Bi2O3 and BiT phases.

Introduction

The Aurivillius family is a class of complex bismuth-based oxides that can be characterized by the general formula (Bi2O2)2−(Am−1BmOm+1)2+ (A = mono-, bi- or tri-valent ion, B = tetra-, penta- or hexa-valent ion and m is the number of perovskite-like layers between the Bi2O2 layers) [1], [2], [3]. Typical examples of compounds belonging to the Aurivillius family are SrBi2Ta2O9, SrBi2Nb2O9, CaBi2Nb2O9(m=2) [4], [5], [6], Bi4Ti3O12(m=3) [7] and BaBi4Ti4O15 (m=4) [8]. In particular, the bismuth titanate (Bi4Ti3O12– BiT) has received special attention due to its potential for the development of non-volatile ferroelectric random access memories (NV-FeRAM’s) [9]. Moreover, the interesting electro-optic properties and the high Curie temperature (675 °C) of this material allow its use in high temperature piezoelectric components, memory storage devices and optical displays [10], [11], [12], [13].

According to the X-ray diffraction data reported by Cummins and Cross [14], the BiT has an orthorhombic structure at room temperature. However, Rae et al. [15] verified through Rietveld refinements that this material presents a monoclinic structure with space group Pc. Recent studies [16], [17] on X-ray and neutron powder diffraction analyses have showed that the BiT is essentially orthorhombic. Kan et al. [18] observed a tetragonal structure for this material below 600 ° C and an orthorhombic structure at 750 °C.

In the last years, different synthesis methods have been reported in the literature for the obtention of BiT powders, mainly including: solid-state reaction [19], high-energy ball milling process [20], [21], self-propagating high-temperature [22], [23], molten salts [24] and flash creation [25]. In contrast, these methods present some drawbacks, such as: formation of secondary phases, contamination by impurities, high heat treatment temperatures, long processing times and nonuniform particle size distribution. A possible alternative for reduction of these problems can be the use of wet chemical methods, such as: sol–gel [26], [27], [28], citrate gel [29], urea precipitation [30], oxalate coprecipitation [31], [32], [33], hydrothermal [34], [35], [36], metalorganic decomposition [37] and polymeric precursor [38], hydrolysis [39]. Moreover, these synthesis methods are able to improve the dielectrical, structural, morphological and mechanical properties of BiT oxides [40], [41], [42].

Currently, the literature has reported few studies on the photoluminescence properties of pure BiT oxides or Eu3+doped [43], [44], [45], [46], [47], [48]. Therefore, in this paper, BiT powders were prepared by the complex polymerization method and heat treated at different temperatures for 2 h under oxygen flow. These powders were structurally characterized by X-ray diffraction (XRD) and micro-Raman (MR) spectroscopy. The optical properties were analyzed by ultraviolet–visible (UV–vis) absorption spectroscopy and photoluminescence (PL) measurements.

Section snippets

Synthesis of Bi4Ti3O12powders

BiT powders were synthesized by the complex polymerization method. In this synthesis, titanium(IV) isopropoxide [Ti(OC3H7)4] (99.9% purity, Aldrich), bismuth nitrate pentahydrate [Bi(NO3)35H2O] (99% purity, Mallinckrodt), ethylene glycol (C2H6O2) (99% purity, J.T. Baker), citric acid (C6H8O7) (99.5% purity, Synth) and ethylenediamine (C2H4(NH2)2) were used as raw materials. Initially, C6H8O7 was dissolved in deionized water at 90 ° C under constant stirring. Afterwards, Ti(OC3H7)4 was quickly

X-ray diffraction analyses

Fig. 1 shows the XRD patterns of BiT powders heat treated at different temperatures for 2  h under oxygen flow.

According to Shuk et al. [49], Bi2O3oxides are able to present four different phases: δ-Bi2O3(face-centered cubic structure), α-Bi2O3 (monoclinic structure), β-Bi2O3 (tetragonal structure) and γ-Bi2O3 (body–centered cubic structure). Rao et al. [50] verified a phase transition from αβ-Bi2O3 between 600 and 735 °C. In Fig. 1(a), all diffraction peaks of BiT powders heat treated at 300 ° C

Conclusions

The BiT powders were synthesized by the polymeric precursor method and heat treated at different temperatures for 2 h under oxygen flow. XRD patterns and Micro-Raman analyses confirmed heat treated at 700 ° C crystallize in an orthorhombic structure without the presence of secondary phases. The heat treatment performed at 300 ° C resulted only in the α-Bi2O3 phase. UV–vis absorption spectra showed that the increase of optical band gap values is associated with the reduction of intermediary energy

Acknowledgements

The authors thank the financial support of the Brazilian research financing institutions: CAPES, CNPq and FAPESP.

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