Particle-shape control and formation mechanisms of hydrothermally derived lead titanate

Phase-pure perovskite lead titanate with various morphologies has been synt hesized by a hydrothermal method at 150 degrees C. Particle shapes include cubic, tabular, and aggregated platelike shapes. The feedstock concentratio n greatly influences particle morphology of the hydrothermally derived PbTi O3. At a concentration of 0.05 M, the tabular particles form while cubic pa rticles are produced at 0.1 M. Aggregated plateletlike particles an synthes ized at 0.125 M. It was observed that both tabular and cubic particles dire ctly precipitate from the coprecipitated precursor gel. In contrast, the pl ateletlike shaped intermediate phase appears during the initial stage of re action at 0.125 M and in situ transforms into perovskite PbTiO3 during furt her hydrothermal treatment. The intermediate phase preserves its particle s hape during transformation and, acting as a template, gives rise to the fin al tabular PbTiO3 particles. It is demonstrated that only base reagents suc h KOH and NaOH, which provide a highly basic condition (i.e., pH > 14), pro mote transformation of the coprecipitated gel into the perovskite PbTiO3. A Hancock and Sharp kinetic analysis in conjunction with microstructural evi dence suggests that the formation mechanism is dissolution and recrystalliz ation in which the degree of supersaturation plays an important role in dic tating the crystallographic particle phase and morphology of the particles. An experimentally constructed solubility diagram indicates that an excess lead condition is necessary to compensate for loss of lead species and to i ncrease supersaturation to expedite precipitation of PbTiO3 at highly alkal ine conditions.