3-D Nanoengineering of Metal Oxides and Oxyhydroxides by Aqueous Chemical Growth
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3-D Nanoengineering of Metal Oxides and Oxyhydroxides by Aqueous Chemical Growth Lionel Vayssieres and Arumugam Manthiram Texas Materials Institute, University of Texas at Austin, Austin, TX 78712 ABSTRACT Advanced nanoparticulate thin films of transition metal oxides consisting of nanorods with different orientations onto various substrates have been successfully grown by aqueous chemical growth without template, surfactant, or applied electric/magnetic field. The synthesis involves the aqueous condensation of metal ions from solutions of metal salts or metal complexes. Such low-cost fabrication of nanoengineered 3-D arrays consisting of 1-D nanorods of iron oxide (hematite), zinc oxide (zincite), and manganese oxyhydroxide (manganite) with parallel and perpendicular orientations onto various substrates are presented. INTRODUCTION Low-cost fabrication of particulate thin films, coatings, and large three-dimensional (3-D) arrays of anisotropic one-dimensional (1-D) nanomaterials such as nanorods, nanowires, and nanotubes is of importance for the development of future nanodevices. In this regard, our strategy to generate large area of advanced nano and micro-particulate thin films at low cost is a bottom-up chemical approach [1] that is well-sustained by a thermodynamic model for the monitoring of the nucleation, growth, and aging processes and the experimental control of the interfacial free energy of the system [2]. Such a strategy has been well-illustrated on the size control of magnetite nanoparticles over an order of magnitude [3]. This concept and synthetic method allows design and creation of metal oxide nanomaterials with novel morphology, texture, and orientation to probe, tune, and optimize their physical properties. Particulate thin films and 3-D arrays are obtained by direct growth onto various substrates from the condensation of aqueous precursors at low temperatures. Such an approach to material synthesis offers the ability to generate anisotropic nanoparticles and to control their orientation onto substrates. Growing and aligning anisotropic nanoparticles into large arrays on a substrate require consideration of the homogeneous and heterogeneous nucleation phenomena. In most cases, homogeneous nucleation of solid phases from solution requires a higher activation energy barrier, and therefore, heteronucleation will be promoted and energetically more favorable. Indeed, the interfacial energy between the crystal and the substrate is smaller than the interfacial energy between the crystal and the solution, and therefore, nucleation may take place at a lower saturation ratio onto a substrate than in solution. Nuclei will randomly appear onto the entire substrate and if their nucleation is controlled and maintained at a limited rate by the precipitation conditions, epitaxial crystal growth will take place from these nuclei along the easy direction of crystallization and a condensed phase of single-crystalline nanorods perpendicular to the substrate will be generated. Alternatively, by enhancing the
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