Particle-shape control and formation mechanisms of hydrothermally derived lead titanate
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Particle-shape control and formation mechanisms of hydrothermally derived lead titanate Jooho Moon, Melanie L. Carasso, Henrik G. Krarup, Jeffrey A. Kerchner, and James H. Adaira) Department of Materials Science and Engineering, University of Florida, Gainesville, Florida 32611 (Received 2 June 1997; accepted 26 June 1998)
Phase-pure perovskite lead titanate with various morphologies has been synthesized by a hydrothermal method at 150 ±C. Particle shapes include cubic, tabular, and aggregated platelike shapes. The feedstock concentration greatly influences particle morphology of the hydrothermally derived PbTiO3 . At a concentration of 0.05 M, the tabular particles form while cubic particles are produced at 0.1 M. Aggregated plateletlike particles are synthesized at 0.125 M. It was observed that both tabular and cubic particles directly precipitate from the coprecipitated precursor gel. In contrast, the plateletlike shaped intermediate phase appears during the initial stage of reaction at 0.125 M and in situ transforms into perovskite PbTiO3 during further hydrothermal treatment. The intermediate phase preserves its particle shape during transformation and, acting as a template, gives rise to the final tabular PbTiO3 particles. It is demonstrated that only base reagents such KOH and NaOH, which provide a highly basic condition (i.e., pH . 14), promote transformation of the coprecipitated gel into the perovskite PbTiO3 . A Hancock and Sharp kinetic analysis in conjunction with microstructural evidence suggests that the formation mechanism is dissolution and recrystallization in which the degree of supersaturation plays an important role in dictating 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 increase supersaturation to expedite precipitation of PbTiO3 at highly alkaline conditions.
I. INTRODUCTION Amorphous particles prepared from solution are generally spherical due to their isotropic growth nature. In contrast, crystalline particles generally assume the intrinsic crystallographic habits associated with the unit cell of the material.1 The Gibbs–Wulff criterion for particle shape states that a polyhedral crystal in equilibrium with the vapor pressure assumes a shape with the minimum surface free energy.2 For an equilibrium crystal shape in a solution, unlike the vapor, the solvent is not merely inert, but assumes some role in the particle formation through which a particular morphology of the particles generates. Solidsolution interactions modify the surface energetics and in turn modify the shape of a crystal produced in a specific solution environment. The shape of a crystal is determined by the difference in relative growth rates of the individual crystal planes, and the resulting particles are normally bound by facets with the lowest growth rate under a certain supersaturation.3 Supersaturation
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