Hydrothermal self-assembling of ZnO nanorods into sphere-like superstructures and their optical characteristics

Self-assembled zinc oxide (ZnO) nanostructures consisting of nanorods were synthesized at low temperature by means of a one-step hydrothermal method using a mixture of hexamethylenetetramine, zinc nitrate [Zn(NO3)2·6H2O], and sodium hydroxide (NaOH). Freestanding single-crystalline ZnO nanorods with polar surfaces on the sharp ends are congregated by the long-range electrostatic force resulting in the formation of ordered nanorod-based structures. The weak Van der Waals interaction causes random aggregation of some tiny nanostructures on the surfaces of the ordered nanostructures. As a result, sphere-like superstructures revealed by X-ray powder diffraction, scanning electron microscopy and transmission electron microscopy are formed due to energy minimization. In addition, some interesting Raman and photoluminescence properties of the composite ZnO nanorod structures are also discussed in detail. Our results are beneficial to a better understanding of the formation mechanism of some ZnO superstructures.