A Generalized Three-Stage Mechanism of ZnO Nanoparticle Formation in Homogeneous Liquid Medium

Understanding the influence of relevant parameters on the kinetics of nanocrystal formation, from reaction and nucleation to the final stages of growth in a homogeneous medium, is crucial in the design and synthesis of nanoparticles of desired dimension. ZnO is chosen as a model system to study spherical nanoparticle formation in solution, using a combination of experiments and mathematical modeling. Stable dispersions of ZnO nanocrystals were synthesized in alcoholic solvents at various temperatures, while site selection between similar to 1.5 and similar to 5 nm diameters were achieved by a rapid-quenching to 253 K, with simultaneous use of a capping agent A priori time scale calculation of the rate processes involving different elementary events, in conjunction with nanoparticle microscopy and spectroscopic data on particle diameter, has led to an underlying three stage mechanism for nanoparticle formation. These sequential stages consist of (I) nucleation of ZnO nanocrystal and growth by molecular diffusion, (II) interparticle coagulative-growth via oriented attachment (OA), and, finally, (III) interparticle growth by Ostwald ripening (OR). Predictions of our mathematical model based on the above mechanism compare favorably with experimental trends on temporal evolution of mean particle diameter, for different alcoholic solvents at, various temperatures in the range of 273-353 K. Solvents of lower carbon chain length and lower temperature independently reduce the rates of both OA and OR facilitating particles of smaller diameter, thereby justifying methanol as the optimum solvent of choice to size-select ZnO nanoparticles in the strong-confinement regime.