Abstract
In order to design and tailor materials for a specific application like gas sensors, the synthesis route is of great importance. Undoped and rhodium-doped barium titanate powders were successfully synthesized by two routes; oxalate route and classic route (a modified conventional route where solid-state reactions and thermal evaporation induced precipitation takes place). Both powders were calcined at different temperatures. X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), energy-dispersive x-ray spectroscopy (EDX) and Brunauer-Emmet-Teller (BET) analyses are employed to identify the phases and polymorphs, to determine the morphology, the chemical composition and the specific surface area of the synthesized materials, respectively. The so-called oxalate route yields pure BaTiO3 phase for undoped samples at 700 °C and 900 °C (containing both cubic and tetragonal structures), while the classic route-synthesized powder contains additional phases such as BaCO3, TiO2 and BaTi2O5. Samples of both synthesis routes prepared by the addition of Rh contain no metallic or oxide phase of rhodium. Instead, it was observed that Ti was substituted by Rh at temperatures 700 °C and 900 °C and there was some change in the composition of BaTiO3 polymorph (increase of tetragonal structure). Heat-treatments above these temperatures show that rhodium saturates out of the perovskite lattice at 1000 °C, yielding other secondary phases such as Ba3RhTi2O9 behind. Well-defined and less agglomerated spherical nanoparticles are obtained by the oxalic route, while the classic route yields particles with an undefined morphology forming very large block-like agglomerates. The surface area of the synthesized materials is higher with the oxalate route than with the classic route (4 times at 900 °C). The presence of the oxalate ligand with its steric hindrance that promotes the uniform distribution and the homogeneity of reactants could be responsible for the great difference observed between the powders prepared by two preparation routes.
Highlights
Barium titanate (BaTiO3) has a typical perovskite structure which exists in a number of polymorphs from high to low temperature: hexagonal, cubic, tetragonal, orthorhombic, and rhombohedral crystal structure
The undoped and Rh-doped BaTiO3 have been prepared by a modified solid-state route and the oxalic coprecipitation route and calcined at different temperatures (700 ◦C, 900 ◦C or 1000 ◦C)
Pure BaTiO3 phase, free of any other secondary phases, is obtained at 700 and 900 ◦C by oxalic route while secondary phases of BaCO3, TiO2 and BaTi2O5 occurs in the undoped powders synthesized by the classic route under the same conditions
Summary
Barium titanate (BaTiO3) has a typical perovskite structure which exists in a number of polymorphs from high to low temperature: hexagonal, cubic, tetragonal, orthorhombic, and rhombohedral crystal structure. Ferroelectric, piezoelectric and optical properties, BaTiO3 is a well-known electro-ceramic material that is used in the manufacturing of thermistors, dielectric ceramic capacitors [4,5,6] and applied as photocatalyst and gas sensor material [7,8] These properties of BaTiO3 are strongly influenced by the existing polymorphs which are linked to the preparation method as well as the dopant. During the last decade many efforts have been devoted to the preparation of metal-incorporated or doped BaTiO3 using different synthesis techniques These can be divided in two categories: (i) solid-state reaction methods and (ii) wet chemistry methods. The synthesis route plays a very important role on the final properties of the materials [15,16,17] since the morphology, particle size, polymorphs and phase compositions are related directly to the synthesis method and reflect the desired functional property of the material
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