Volume 22 Issue 1 - April 13, 2012 PDF
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Heterostructures of ZnO–Zn coaxial nanocables and ZnO nanotubes
Department of Chemical Engineering, College of Engineering, National Cheng Kung University
 
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Growth of one-dimensional (1D) ZnO nanostructures including nanowires, nanorods, and nanobelts have attracted much attention because ZnO is a versatile material.  In this letter we reported on the formation of the heterostructures of ZnO–Zn coaxial nanocables and ZnO nanotubles.  The tubular ZnO nanostructure is demonstrated for the first time.

The heterostructures of Zn–ZnO coaxial nanocables and ZnO nanotubes were synthesized by simple pyrolysis of zinc acetylacetonate in a two-temperature-zone furnace. Zinc acetylacetonate hydrate placed on a cleaned silicon susceptor was loaded into the low temperature zone of the furnace which was controlled at 130–140 °C to vaporize the solid reactant. The evaporated gaseous species were carried by a 500 sccm N2 flow at a total pressure of 200 Torr into the higher temperature zone of the furnace which was set at 500 °C. Black products were formed on the wall of the quartz tube in the region down stream out of the higher temperature zone with a temperature of 230 °C.

Figure 1a shows the SEM image of the 1D ZnO nanostructures with an average diameter of 30 nm.  Typical TEM image, as shown in Figure 1b, reveals that some tubular nanostructures are observed among the nanowires.  The nanowire possesses a coaxial structure, that is, a thin sheath with lighter contrast is formed outside the surface of nanowire-like structure of dark contrast.  Moreover, some of the tubular nanostructures are connected with the coaxial nanocables.  EDS analyses reveal that the coaxial nanocable is composed of the Zn core and the ZnO sheath. As shown in Figure 2, high-resolution TEM analyses reveal that the Zn core and the ZnO sheath of the nanocables have an epitaxial relationship with their longitudinal axis oriented along the <001> direction.  ZnO nanotubes with a wall thickness of 4 nm possess a single-crystal structure and appear to be the extension of the ZnO sheath of the coaxial nanocables. It is suggested that the ZnO nanotubes are formed by partial evaporation of Zn core of the Zn–ZnO coaxial nanocables.  The Zn nanowires could be considered as the templates for subsequent spontaneous formation of single crystalline ZnO nanotubes.
Figure 1 (a) SEM image of the 1D ZnO nanostructures. (b) TEM image of the heterostructures of the coaxial nanocables and the nanotubes.

Figure 2 High-resolution TEM image of the heterostructure of coaxial nanocable and nanotube.
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