Two-stage crystal-growth kinetics observed during hydrothermal coarsening of nanocrystalline ZnS

Crystal growth during hydrothermal coarsening of mercaptoethanol-capped nanocrystalline ZnS occurs via a two-stage process. In the first stage, the primary particle quickly doubles in volume. The initial growth rate can be fitted by an asymptotic curve that cannot be explained by any existing power-law dependence kinetic model developed for more coarsely crystalline material. High-resolution transmission electron microscope (HRTEM) data indicate that crystal growth within spherical nanoparticle aggregates occurs via crystallographically specific oriented attachment, despite the presence of surface-bound organic ligands. The size stabilizes for a period of time that depends on the coarsening temperature. In the second stage, following the dispersal of nanoparticles, an abrupt transition from asymptotic to cubic parabola growth kinetics occurs. The crystal growth data can be fitted by a standard Ostwald ripening volume diffusion model consistent with growth controlled by the volume diffusion of ions in solution. However, HRTEM data indicate that oriented attachment-based growth occurs in the early part of the second stage, followed by a significant reduction in aggregate surface topography, probably via surface diffusion as well as volume diffusion. We propose a new kinetic model based on oriented attachment-based growth to explain the asymptotic growth in the first stage of coarsening. The presence of surface-bound organic ligands may control the aggregation state of the nanoparticles and may permit an almost exclusive crystallographically specific oriented attachment-based growth to dominate in the first stage.