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Cobalt oxide-tungsten oxide nanowire heterostructures: Fabrication and characterization Nitin Chopra,1,2,* Yuan Li,1 Kuldeep Kumar1 1

Metallurgical and Materials Engineering Department, Center for Materials for Information Technology (MINT), The University of Alabama, Tuscaloosa, AL 35487, U.S.A. 2 Department of Biological Sciences, The University of Alabama, Tuscaloosa, AL 35487, U.S.A. *Corresponding Author E mail: [email protected], Tel: 205-348-4153, Fax: 205-348-2164 ABSTRACT Nanowire heterostructures comprised of cobalt oxide and tungsten oxide were fabricated in a core/shell configuration. This was achieved by sputter coating tungsten oxide shells on standing cobalt oxide nanowires on a substrate. To ensure the polycrystallinity of tungsten oxide shell, the nanowire heterostructures were subjected to post-sputtering annealing process. The cobalt oxide nanowires for this study were grown employing a thermal method via vapor-solid growth mechanism. The crystal structures, morphologies, dimensions, and phases at various growth stages of nanowire heterostructures were studied using high resolution electron microscopy, energy dispersive spectroscopy, and X-ray diffraction methods. The interfaces of these nanowire heterostructures were also studied and showed variation in the lattice spacing across the heterostructure diameter. Results indicated that the cobalt oxide nanowires survived multiple processing steps and resulted in stable heterostructure configurations. The investigation shows, for the first time, a dry processing route for the formation of such novel nanowire heterostructures. INTRODUCTION Nanowire heterostructures are novel 1-D nanoarchitectures that can exhibit unique surface functionality, chemical and electrical properties, and light-matter interactions [1,2]. For photocatalysis, the aim is to develop 1-D geometry with direction-dependent light-matter interactions [3,4]. For example, 1-D geometry such as core/shell nanowires allow for greater absorption of light in longitudinal direction and result in efficient charge transport (or charge carrier generation) in radial direction [4,5]. However, this is dependent on energy and crystal structure, morphology, size, and material selection for the core/shell nanowires. In addition, one of the challenges for photoactive nanostructures is to eliminate or mitigate the use of noble metal systems and explore cheaper oxides [6]. In this regard, the oxides of cobalt and tungsten are of interest due to stability of these oxides, ability to result in tunable and size-dependent band gap energies, and can be fabricated through a variety of approaches [4,7,8]. Tungsten oxide is a visible light active catalyst (band gap energy ~2.8 eV) when sacrificial agents or precious metals are present and cobalt oxide is a p-type semiconductor (band gap energy ~1.6 eV) and of potential interest for photocatalysis as well as batteries [4,9,10]. The major goal of this research was to develop novel oxide-based nanowire heterostructures with controlled morphology, chemical composi