Monodispersed and Well-Crystallized Zinc Oxide Nanoparticles Fabricated by Microemulsion Method

Monodispersed and spherically shaped zinc oxide (ZnO) nanoparticles with particle sizes of < 5 nm were successfully fabricated by the microemulsion (ME) method. In this process, ZnO nanoparticles were formed at room temperature by the hydrolysis reaction of zinc diethoxide in the small water droplets in reverse micelles. Electron diffraction patterns and lattice images demonstrated that the obtained ZnO nanoparticles were composed of well-crystallized single crystallites having a wurtzite structure. Further, we succeeded in controlling the average particle sizes in the range of 3.9-2.4 nm by changing the surfactants. The bandgap energies of the ZnO nanoparticles estimated from the derivative curve of the optical absorption spectra were 3.65 eV (3.9 nm), 3.66 eV (2.9 nm), 3.69 eV (2.7 nm), and 3.72 eV (2.4 nm). The bandgap energies were found to be considerably higher than the bandgap energy of the ZnO bulk (=3.37 eV). Thus, the quantum size effect in the ME-derived ZnO nanoparticles was successfully observed in this study. The photoluminescence spectra of the ZnO nanoparticles exhibited intense green luminescence at an energy of around 2.5 eV.