Nucleation and Growth of Gallium Oxide Tubes, Nanopaintbrushes and Nanowires from Molten Gallium
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Nucleation and Growth of Gallium Oxide Tubes, Nanopaintbrushes and Nanowires from Molten Gallium Shashank Sharma and Mahendra K. Sunkara* Department of Chemical Engineering, University of Louisville, Louisville, KY 40292, U.S.A Uschi M. Graham and Burtron H. Davis Center for Applied Energy Research, University of Kentucky, Lexington, KY 40506, U.S.A * Corresponding author: [email protected] ABSTRACT We have synthesized highly crystalline β-gallium oxide tubes, nanowires, and unique onedimensional structures in the form of nanopaintbrushes using molten gallium and microwave plasma containing a mixture of monoatomic oxygen and hydrogen. Multiple nucleation and growth of gallium oxide nanostructures occurred directly out of molten gallium upon exposure to an appropriate composition of hydrogen and oxygen in the gas phase. Gallium oxide nanowires were 20 to 100 nm thick and tens to hundreds of microns long. In addition to these morphologies, we also report for the first time, non-template based synthesis of novel 2-D networks of crystalline gallium oxide nanowires and nanotubes. Demonstration of this technique with gallium oxide certainly presents a new route for synthesis of nanostructures of other important metal oxides such as indium oxide, tin oxide, and zinc oxide. INTRODUCTION Gallium oxide is a wide energy band gap material (4.9 eV) with interesting conduction and luminescence characteristics. 1,2 These characteristics make gallium oxide nanostructures particularly interesting for optical and optoelectronic device applications. 3 In addition, gallium oxide nanostructures will be useful as high temperature stable sensors and catalysts. 4-8 We recently reported a conceptually novel synthesis technique for one-dimensional nanostructures of gallium oxide using a direct reaction of molten gallium with oxygen and hydrogen radicals in an activated vapor phase. 9 In this technique, nanometer scale nuclei form in the bulk pool of the molten metal that segregate to the surface and grow in one dimension upon basal attachment. Here, we report our results with the synthesis of novel one-dimensional morphologies in gallium oxide nanostructures. We demonstrate that the plasma chemistry directly influences the nanostructure morphology. In addition, we report for the first time, non-template based synthesis of gallium oxide nanotubes and 2-D crystalline networks. Our approach of direct synthesis of metal oxide nanostructures can be easily extended to other low-melting metals. EXPERIMENTAL DETAILS The synthesis was carried out in a microwave plasma reactor (ASTEX 5010) using H2/O2 gas mixtures. Quartz, alumina, pyrolytic boron nitride, glassy carbon, polycrystalline diamond
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film, porous graphite, and sapphire substrates were covered with a thin film of molten gallium and were exposed to a microwave plasma containing a range of gas phase species. The substrate temperature was measured using an infrared pyrometer to be approximately 550 °C for 700 W microwave power, 40 Torr total pressure, and 8.0 sccm O2 i
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