Mesoporous single-crystal nanowires

Porous materials have a wide variety of applications in bioengineering, catalysis, environmental engineering, and sensor systems because of their high surface-to-volume ratio. Normally, most of these mesoporous structures are composed of amorphous materials and porosity is achieved by solvent-based organic or inorganic reactions. There are few reports of mesoporous structures based on crystalline material. We have reported a novel wurtzite ZnO nanowire structure that is a single crystal but is composed of mesoporous walls/volumes. The synthesis is based on a modified solid-vapour process. ZnO nanowires grown on a Si substrate coated with a thin layer of Sn catalyst, the typical length of the nanowires varies from 100 μm to 1 mm and the diameter mask and self-assembled submicron spheres. We have combined this self-assembly-based mask technique with the surface epitaxial approach to grow large-area hexagonal arrays of aligned ZnO nanorods. The synthesis process involves three main steps. The hexagonally patterned ZnO nanorod arrays are grown on a single-crystal Al2O3 substrate on which patterned Au catalyst particles have been dispersed. First, a two-dimensional, large area, self-assembled and ordered monolayer of submicron polystyrene spheres is introduced onto the single-crystal Al2O3 substrate. Second, a thin layer of Au particles is deposited onto the self-assembled monolayer; the spheres are then etched away, leaving a patterned Au catalyst array. Finally, nanowires are grown on the substrate using the VLS process. The spatial distribution of the catalyst particles determines the pattern of the nanowires. This step can be achieved using a variety of mask technologies for producing complex configurations. By choosing the optimum match between the substrate lattice and the nanowires, the epitaxial orientation relationship is in the range of 50-500 nm. The porous structure of ZNO:

(0001) zno || (001) zn2sio4, [2110]zno||[100]zn2sio4

A corresponding electron diffraction pattern from the nanowire presents two sets of structures.
the formation of Zn2SiO4 on the surface of the nanowires with an epitaxial orientation relationship as follows, Zn2SiO4 is formed on the surface of the Si substrate, but covers only a fraction of the surface area because of the large lattice mismatch with ZnO. As a result, resublimation of ZnO in the nanowire forms the mesoporous structure. The high porosity, single-crystal wire-like structures have potential applications as filters, catalyst supports, and gas sensors.

In ZNO the nanostructures can have novel applications in optoelectronics, sensors, transducers, and biomedical science because it is bio-safe.

Wang, Nanostructures of Zinc Oxide, Materials Today, Volume 7, Issue 6, June 2004, Pages 26-33

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