Kinetic and Thermodynamic Aspects in the Microwave-Assisted Synthesis of ZnO Nanoparticles in Benzyl Alcohol

A detailed study of kinetic and thermodynamic aspects in the microwave-assisted synthesis of ZnO nanoparticles from zinc acetate and benzyl alcohol is presented. The use of a nonaqueous sol-gel approach provides the unique opportunity to investigate simultaneously the organic reaction, that is, the esterification between acetate and benzyl alcohol, and the inorganic process, represented by the growth of the ZnO nanoparticles. Monitoring both the formation of the organic species as well as ZnO crystal size with time makes it possible to directly correlate the kinetics of the organic side reaction with the growth kinetics of the ZnO nanoparticles. The esterification reaction, which is the chemical basis for producing the monomers for ZnO formation, was found to be first order. The growth of the ZnO nanoparticles followed the Lifshitz-Slyozov-Wagner model for coarsening, pointing to a diffusion-limited process. Comparison of the microwave-mediated route with conventional heating showed that microwave irradiation greatly accelerates nanoparticle formation by (a) facilitating the dissolution of the precursor in the solvent, (b) increasing the rate constants for the esterification reaction by 1 order of magnitude, resulting in faster production of monomer and consequently in an earlier nucleation event, and (c) increasing the rate constants k(growth) for the crystal growth from 3.9 nm(3)/min (conventional heating) to 15.4 nm(3)/min (microwave heating).