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 rain, 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(EOb-SSH) copolymers favor the rapid nucleation of the highly ordered lamellar intermediate.