Mechanisms of Hydrothermal Barium Titanate Thin Film Formation
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It appears from the discussion above that there are conflicting explanations in the litrature and that there is a need for clear understanding of the formation mechanism of BaTiO 3 particles and thin films during the reaction between titania precursors and barium hydroxide solution. In order to realize the evolution of the particle formation, it is necessary to device an observation technique by which the formation mechanism is investigated at all stages from nucleation to growth that allows the identification of reactants and products during the course of BaTiO 3 formation. EXPERIMENTAL PROCEDURES Titania soot (Typ P-25, Degussa Corp.) and a clear solution of 1M Ba(OH)2 .8H 20 (Baker Chemicals) was used with a Ti:Ba ratio of 1:1.2. The reaction took place at 80*C in sealed polyethylene bottles without stirring. Each suspension was washed to remove unreacted Ba(OH) 2 solution using C0 2 -free water by centrifugation. The sequence of events in terms of structural evolution and compositional changes during the reaction series that leads to the formation of BaTiO 3 particles were investigated on samples that were removed from the suspension at various time intervals between 10 minutes to 48 hours. To accomplish this task, a drop of suspension at each time interval was placed on a holey carbon film and examined with a high resolution electron microscope (HREM). For HREM, a Philips 430T (Philips Electronic Instruments Inc., Mahwah, NJ) at 300 kV equipped with a parallel detection electron energy loss spectrometer (PEELS, GATAN 666) and an energy dispersive x-ray spectrometer (EDS, EDAX Co.) was used for which the samples were placed in a single-tilt holder. Thin film formation of BaTiO 3 was observed on three different substrates (i) polystyrene spin-coated by sol-gel method using titanium diisopropoxide bis(2,4-pentanedionate); (ii) titanium metal; and (iii) sintered polycrystalline TiO 2 . The hydrothermal reaction conditions and processing steps were similar to the particle formation as described above. The excess barium hydroxide on the surface was removed by rinsing with C02-free water. The morphology and composition of the thin films were examined by electron microscopy techniques. RESULTS AND DISCUSSION Particle formation during hydrothermal reactions is affected by experimental parameters such as temperature, pressure, concentration of the reactants, time and the degree of mechanical mixing of the components within the suspension. The formation mechanism of BaTiO 3 particles was studied at different time intervals [16]. Upon the addition fairly monosize (150 A dia.) particles of TiO 2 , small cubical BaTiO3 particles (few nanometers in edge length) and an amorphous phase appear in the vicinity of the TiO 2 particles within 30 minutes (B, A and T, respectively, in Figure Ia). At this stage of reaction, the cubic-shaped particles are single crystalline when they are small and they stay as such even though they grow to larger sizes with ragged edges, as evidenced from the HREM image in Figure lb. These particles con
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