Microstructure development in zinc oxide nanowires and iron oxohydroxide nanotubes by cathodic electrodeposition in nano
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The cathodic electrodeposition of crystalline ZnO nanowires and amorphous FeO(OH) nanotubes in polycarbonate track-etched membranes with pore diameters of 50–200 nm is reported. Nitrate was used as a sacrificial precursor for the electrochemical generation of hydroxyl ions that raised the pH of the interior of the nanopore, leading to precipitation of a metal oxide or hydroxide phase. The crystalline and semiconducting ZnO phase formed directly above 60 °C at sufficiently high pH and led to the formation of dense nanowires with preferential (0001) orientation. The morphology of the wire could be influenced by the deposition temperature. Axially segmented gold–ZnO and silver–ZnO nanowires were made. In contrast, the iron hydroxide phase deposited inside the pore as a permeable gel that collapsed and transformed into hollow FeO(OH) tubes during drying. The as-formed nanotubes were amorphous and could be filled with nickel in a subsequent electrodeposition step, yielding core-shell nickel iron-oxohydroxide nanowires. The cathodic efficiency of nitrate reduction was low in both cases, suggesting that diffusional supply of metal ions may be the rate-determining step.
I. INTRODUCTION
One-dimensional (1D) nanostructures such as nanowires and nanotubes have been the focus of extensive research.1 Because of their large surface-to-volume ratio, they are important building blocks for various nanotechnological applications, e.g., when surface sensitivity is required. Nanowires and nanotubes are the active components in nanosensors2–5 for measuring molecules or gases in concentrations of nanomolars or lower, in optical or molecular tags6–8 for cell tracking applications, and in self-propelling nanomotors.9–11 Nanowires and nanotubes may also enable development of new technologies in energy conversion and storage technologies, e.g., novel battery architectures.12 A variety of synthesis techniques have been developed, such as vapor-based growth techniques like the Vapor–Liquid–Solid method for nanowires13,14 and liquid-based templated growth techniques for nanowires and nanotubes.15 Several types of oxide nanowires and nanotubes, e.g., ZnO,16–23 Ni(OH)2,24 and TiO2,25 have been made by cathodic electrodeposition in nanopores, in which a sacrificial precursor is electrochemically transformed into hydroxyl ions that raise the pH above the solubility limit of the metal. Anodic alumina16–18,23 and polycarbonate a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2011.93 J. Mater. Res., Vol. 26, No. 17, Sep 14, 2011
track-etched (PCTE) membranes19–22 have been used to guide the growth of the oxide phase. Both systems have a high density of pores with straight channels of uniform diameter.15 In view of our interest in fabricating nanowires and nanotubes with a complex architecture and composition, such as segmented nanowires and core-shell morphologies, it is important to have detailed insight in the way in which the oxide phase grows morphologically in straight-channel nanopores. In this study
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