Sequential occurrence of ZnO nanopaticles, nanorods, and nanotips during hydrothermal process in a dilute aqueous solution

The ZnO nanostructures have been synthesized by hydrothermal process in a dilute aqueous solution consisting of Zn(AC)2 and NaOH. X-ray diffraction (XRD) pattern indicates that the synthesized ZnO crystal is of hexagonal phase. By means of transmission electron microscopy (TEM) and field emission scanning electron microscopy (FE–SEM), it has been revealed that the ZnO nanoparticles, nanorods and nanotips sequentially occur along with the hydrothermal process, during which the concentration of Zn source is ever decreasing. The nanoparticles terminated with pyramids firstly generate at the early stage of hydrothermal process; subsequently, a number of seed-crystals branch on the pyramids, furthermore, the homo-epitaxy based on which along the <0001> direction results in the formation of nanorods; finally, due to the extremely low concentration of Zn source, instead of the continual elongation of the nanorods, the nanotips dispersively occur on the top flat plane of nanorods as a result of selective epitaxy.