The nucleation kinetics of ZnO nanoparticles from ZnCl2 in ethanol solutions

The first stages of the synthesis of ZnO nanoparticles by forced hydrolysis of ZnCl2 with NaOH and water in ethanol have been investigated using UV-Vis spectrophotometry. At sufficiently low water concentrations, focusing of the nanoparticle size distribution was observed during the nucleation and growth phase, followed by a defocusing phase when coarsening becomes significant. During nucleation and growth, only the smaller particles grow while the larger particles have an essentially zero growth rate, indicating that the growth rate decreases rapidly with particle size. As the average particle size remains nearly constant in this regime, the absorbance increase with time can be used to determine the nucleation rate. The nucleation rate was found to depend on both the water concentration and the reactant concentrations. The results are discussed in terms of a mechanism where water determines the precursor formation kinetics thus controlling the nucleation rate.