Self-organized Zn/ZnO core-shelled hierarchical structures prepared by aqueous chemical growth
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J.M. Chou Department of Materials Science and Engineering, I-Shou University, Kaohsiung Hsien 840, Taiwan, Republic of China
I.C. Leua) Department of Materials Science and Engineering, National United University, Miao-Li 360, Taiwan, Republic of China
M.H. Hon Department of Materials Science and Engineering, National Cheng Kung University, Tainan 701, Taiwan, Republic of China (Received 10 October 2007; accepted 17 January 2008)
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.
I. INTRODUCTION
The various nanostructures of semiconductor materials will have numerous potential applications in a variety of fields. Novel three-dimensional (3D) hierarchical nanoarchitectures of materials may find applications that require not only high surface area but also structural integrity. The synthetic approaches can be generally classified into two categories: vapor-phase growth and solutionphase growth. In the vapor-phase growth system, the hierarchical structure obtained by the vapor-liquid-solid (VLS) technique combining with metal catalyst (or sometimes catalyst free) are usually adopted.1,2 A simple solution-based process has also been developed for the formation of 3D hierarchical structures, but organic structure-directing agents are commonly chosen as templates.3 Therefore, fabricating symmetrical nanostructures with self-assembly in a controlled way and the elucidation of the mechanism of nucleation and growth are a
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2008.0146 J. Mater. Res., Vol. 23, No. 4, Apr 2008
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challenge for the developments of nanoscience and nanotechnology. This work may provide a new rationale pertaining to the design and preparation of ordered nanostructures by a simple solution method. Among the approaches reported for preparing ZnO nanostructues, the employment of metallic foil as the substrate is a simple and convenient method.4,5 Herein, the formation of hierarchical structure is performed by one-step aqueous chemical growth on microstructured Zn foils. Then, the hierarchical structures are composed of a pyramid microstructure Zn, interface of Zn/ZnO, and ZnO branching nanostructures. II. EXPERIMENTAL PROCEDURE
Different substrates, including polycrystalline (Zn foil) and single-crystalline microstruc
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