Electrosynthesis and Microstructural Characterization of Anodic VO x Films
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B.R. Johnson and W.M. Kriven Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801 (Received 30 September 1999; accepted 27 March 2000)
Anodic conversion films of vanadium oxides on vanadium were potentiodynamically generated at high voltages in an acetate electrolyte system. The microstructure of the anodic VOx coatings was characterized by surface and solid-state techniques such as scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, and x-ray photoelectron spectroscopy. An amorphous structure is proposed in which network-forming [VO4] tetrahedra in various degrees of condensation are connected by distorted [VO5] and [VO6] units. Such polyhedra lead to the formation of nanocrystalline phases of stoichiometric and substoichiometric vanadium oxides, which were observed in the amorphous phase.
I. INTRODUCTION
Anodic films on transition metals are of widespread scientific and commercial interest because of their unique physical and chemical properties. They are used in numerous high-technology areas including electronic devices, adhesive bonding, corrosion resistance, or advanced biomaterials.1–3 Many researchers have studied the structure and growth mechanisms in anodic films, in particular porous alumina on aluminum metal.1,4 However, the complex structure–property relationships are still not well understood. In comparison with the more widely investigated anodic film-forming metals such as tantalum or aluminum, studies of the structure of anodic conversion films on vanadium, deposited in aqueous and nonaqueous systems, have rarely been published. There are several different ways to prepare native oxide films on vanadium metal, in addition to anodic deposition. For example, coating techniques such as sol-gel spin coating, flash evaporation, and vacuum or electron beam evaporation are increasingly used for the fabrication of vanadium oxide films, because of their potential applications such as catalysts, electrochromic devices, or high-energy density lithium microbatteries.5–8 The first published attempts to prepare anodic vanadium oxides in acetic acid solutions and to identify the anodic conversion films formed were reported by Keil and Salomon.9 The films were studied by attenuated total reflectance spectra, and the results were characteristic of V2O4 structures. Extended investigations of the microstructure and stoichiometry of anodic films on vanadium were published by Arora and Kelly.10 The anodic films J. Mater. Res., Vol. 15, No. 7, Jul 2000
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were generated at voltages of 4 and 6 V, respectively. Reflection electron diffraction (REED) examinations of films exhibited very weak diffraction patterns of an unidentified oxide. At voltages higher than 6 V, amorphous films were produced. Hornkjoel,11 who used chemical analysis and x-ray photoelectron spectroscopy (XPS) to show that the oxide film contained pentavalent vanadium, studied the anodic behavior of vanadium
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