Influence of synthesis route on the formation of ZnO particles and their morphologies

The formation and microstructure of ZnO particles, obtained by mixing concentrated aqueous Zn(NO3)2 and NaOH solutions, were monitored by X-ray powder diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, field emission scanning electron microscopy (FE-SEM) and specific surface area measurement (BET). At 160 °C and a pH near 6, plate-like Zn5(OH)8(NO3)2(H2O)2 particles were formed at the beginning of the precipitation process, then quickly transformed into ZnO via a dissolution/reprecipitation mechanism. ZnO particles of different geometrical shapes, based on the hexagonal prism, were produced. Precipitation at 20 °C and pH near 6 yielded plate-like Zn5(OH)8(NO3)2(H2O)2 particles which were present up to 6 months of aging. Autoclaving of the precipitation system at 160 °C up to 72 h and a pH near 7 yielded only ZnO particles as pseudospheres, hexagonal dipyramids and an open form of the hexagonal pyramid. All these particles were made up of much smaller ZnO units. Only ZnO particles precipitated at 160 or 20 °C and a pH near 13. All ZnO particles, thus obtained, were plate-like and their size depended on the temperature and time of aging. BET measurements can be related with the size of ZnO particles, as inspected with FE-SEM. The FE-SEM inspection did not show any formation of microporous ZnO particles. The spectral shape of the corresponding FT-IR spectra can also be related to the geometrical shapes and size of ZnO particles.