Anodizing of metals and alloys is a simple electrochemical process to form various nano-structured oxide films, including self-ordered nanoporous and nanotubular films. Typical examples are nanoporous alumina films with honeycomb morphology and nanotubular array of anodic titanium oxide. Recently, several nanowire-type anodic films have been prepared on metals, including aluminum, copper and zinc. The formation mechanism of of the nanowire films may be different from those of the nanoporous and nanotubular anodic films. Here, our recent studies on the formation, characterization and applications of the anodic nanowire films on metals are presented.

When zinc plate was anodized at a constant voltage in KHCO₃ aqueous electrolytes, nanowires were covered on the entire zinc surface. From the SEM observations, it was found that pits were initially formed on zinc surface. The low-voltage SEM observations disclosed that some small pits were covered with a thin passive film, and in such pits nanowires were preferentially nucleated probably because of the increased concentration of dissolved Zn²⁺ ions in the pits. Then, the nanowires were grown radially over the pits and covered the entire zinc surface.

The nanowires were not composed of ZnO. XPS and FT-IR analyses of the nanowire films indicated the presence of carbonate and hydroxide species; the nanowire consisted of basic zinc carbonate. TEM observations confirmed the single crystalline nature of the as-formed nanowires. The basic zinc carbonate was converted to ZnO without changing the nanowire morphology by heat treatment in air, although single crystalline nanowires changed to polycrystalline one. In addition, slit-type nanopores were introduced into the nanowires so that the specific surface area of the nanowire films was highly enhanced by the heat treatment.

The nanowire films exhibits unique wettability to liquids. The as formed nanowire films showed superhydrophilicity with a water contact angle close to 0^o and after organic coating the surface changed to superhydrophobic with a water contact angle greater than 150^o. Such surface may have many potential applications.