Formation mechanism of radial mesocrystals consisting of ZnO nanowires

Abstract

Radial semiconductor mesocrystals (rad-SEM MCs), the so-called sea urchin-like microspheres, combining both the properties of nanoparticles and microparticles have widely attracted much interest. Elucidation of the detailed formation mechanism is a difficult but scientifically important challenge underpinning the enhancement of applications. In this case, both the mechanisms of the growth of the building blocks (classical crystal growth) and their self-assembling (non-classical crystal growth) should be clarified. In this study, rad-ZnO MCs consisting of tapered ZnO nanowires (NWs) were synthesized from a mixed solution of 25 mM Zn(NO₃)₂ and 25 mM hexamethylenetetramine under low supersaturation conditions. The time change of the products was traced in detail by means of various analytical tools. On the basis of these results, we have clarified that the formation of rad-ZnO MCs is initiated from the generation of ZnO nuclei inside an amorphous Zn(OH)₂ phase followed by preferential zone-axis growth to ZnO NWs, and self-assembling of linear ZnO NW dimers. Moreover, the formation of the tapered ZnO NWs originates from the preferential crystal growth in the [0001] direction with many (10−11) steps, and the self-assembling of the anisotropic ZnO NWs yields rad-ZnO MCs due to the balance between the interparticle van der Waals attractive force and electrostatic repulsive force.