Kinetics and Particle Formation Mechanism of Zinc Oxide Particles in Polymer-Controlled Precipitation from Aqueous Solution

The morphogenesis, particle size, size distribution, and phase evolution of zinc oxide precipitated in the presence of water-soluble poly(ethylene oxide- block -methacrylic acid) (P(EO- b -MAA)) and poly(ethylene oxide- block -styrene sulfonic acid) (P(EO- b -SSH)) diblock copolymers is reported. Without a polymeric additive, spindlelike particles with a central grain boundary form along with multiply twinned particles. After ∼30 min, the multiple twins are gone, small, needlelike crystals appear, and the sample becomes more polydisperse. With P(EO- b -MAA) copolymers, initially more rounded particles with the same central grain boundary and a narrow size distribution form. They preferentially grow along the crystallographic c -axis and eventually adopt a hexagonal prismatic shape. With P(EO- b -SSH) copolymers, a lamellar intermediate precipitates first. It eventually dissolves and hexagonal prismatic crystals form; in a subsequent growth process unique to these polymeric additives, the crystals grow along the crystallographic a -axis and transform to another morphology termed the “stack of pancakes” shape. Both in the absence of polymer and with P(EO- b -MAA) copolymers, multiple particle generations precipitate. With P(EO- b -SSH) copolymers, no second generation is observed. Nucleation is delayed by the P(EO- b -MAA) copolymers, while P(EO- b -SSH) copolymers favor the rapid nucleation of the highly ordered lamellar intermediate.