Metal Oxide Nanowire Growth via Intermediate Hydroxide Formation:A Thermochemical Assessment
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Metal Oxide Nanowire Growth via Intermediate Hydroxide Formation: A Thermochemical Assessment Avi Shalav and Robert G. Elliman Department of Electronic Materials Engineering, Research School of Physics and Engineering, Australian National University, Canberra, ACT0200, Australia
ABSTRACT In this study we apply reaction thermodynamics to show that a significant volatile hydroxide vapor partial pressure forms at a metal-oxide interface and is a likely precursor source for nanowire growth. The growth of WO3 and CuO nanowires are used as examples for reactions dependent on only H2O and O2+H2O, respectively. Optimal temperatures, H2O (and O2) partial pressures for volatile hydroxide formation are calculated and experimentally investigated. We conclude that metal oxide nanowires can be readily grown at relatively low temperatures (close to or less than 500oC) over short anneal times (tens of minutes). The growth of these metal oxide nanowires, with many oxidation states, by this simple thermal technique is readily suited for a range of emergent large surface area nanostructured optical and electrical applications, including sensing, photocatalysis and ultracapacitors. INTRODUCTION Many transition metals, for example Fe, Cu, V, Zn, Al, W and Mo, have been shown to readily form oxide nanowires upon annealing at elevated temperatures under atmospheric conditions [1]. The growth mechanism is widely accepted to be dependent on metallic-oxygen reactions with atmospheric O2, where metal cations diffuse along the grain boundaries from the metal-oxide interface to the surface. The role of H2O vapor is often neglected, even though it is well known within corrosion science that metals readily oxidize in the presence of H2O vapor and that one or more volatile hydroxide species can be produced [2]. The high temperature oxidation of metals under O2 and/or H2O vapor conditions, although well researched, is particularly complex due to the formation of a number of stable and unstable oxide scales. For nanowires to grow via a vapor-liquid-solid (VLS) or a vapor-solidsolid (VSS) mechanism, a vapor transport precursor is required. Evidence suggests that if an underlying substrate is to provide this precursor, a metastable surface oxide is required to separate catalytic metal particles from the substrate [3-5] and/or to provide nucleation sites for diffusion mediated growth [6, 7]. The aim of this study is to investigate the annealing conditions required to grow metal oxide nanowires directly from a metal substrate, where the substrate itself provides the required precursor for nanowire growth. Two different metals have been selected and thermochemically investigated to illustrate slightly different growth conditions, namely H2O and O2+H2O dependencies. Firstly, W and its oxides are both unstable at higher temperatures in the presence of H2O vapor producing the volatile hydroxide WO2(OH)2 [6, 8] . Although oxide formation
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depends on the amount of O2 present, the production of WO2(OH)2 is independent of the O2 partial pressure, that is
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