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Six-fold core-shelled three-dimensional hierarchical structures were prepared by a direct annealing process. The effect of experimental parameters on density and length of branches of hierarchical Zn/ZnO structure is discussed. The influence of oxide layer formation and the shape of Zn polyhedron on the relaxation of stress for nanostructure growth also is reported. At the annealing temperature of 375 °C, the largest aspect ratio (∼71) of branched nanostructures and the smallest density (0.05 tips/μm2) of hierarchical arrays can be obtained. Transmission electon microscopy analysis shows that the oxide layer grows epitaxially from the Zn microtips and then the branches grow epitaxially from the oxide layer. For field emission applications, a better turn-on electric field (8.5 V/μm) and a larger enhancement factor 3490 are obtained for branched nanostructures with the high aspect ratio.

Self-assembled core-shelled hierarchical structures consisting of single-crystalline pyramid Zn microtip as a core, converted ZnO coating as the shell, and the grown ZnO nanowires as branches, have been prepared. Such ZnO hierarchical structures fabricated by a simple aqueous chemical growth method on Zn foil substrate are expected to be easily integrated into nanodevices. These self-organized structures are superior to both the random nanoarchitecture arrays formed in vapor system and the precipitated nanostructures suspended in the solution. Because of the easier transportation of electrons from the metallic core to ZnO branches, the self-assembled core-shelled hierarchical structures exhibit better field-emission characteristics.