(Invited) Anodizing of Metals for Fabrication of Functional Nanostructured Surfaces
Authors: Habazaki Hiroki 
Published: December 2020

Anodizing of metals, including aluminum, titanium, and iron, is a simple and const-effective process to form various nanostructured oxide films, such as nanoporous, nanotubular, and nanowire films. Such nanostructured anodic films have attracted much attention for many potential functional applications as well as fundamental interests on the mechanism of self-organized nanostructure formation. Here, our recent studies on the formation of nanostructured anodic films on aluminum, copper, and zinc and their application to controlled wettability will be outlined.
Anodizing of zinc in KHCO3 electrolyte produces a nanowire-type anodic film on entire metal surface. The formation of the nanowire film involves the initial pitting corrosion; pits are developed on the zinc metal surface, and embryos of nanowires are formed within the pits. When the pits are covered with a thin air-formed or electropolishing film, the nanowire formation is promoted as a consequence of the increased concentration of dissolved zinc species within the pits. Then, the nanowires grow toward the outside of the pits and cover the entire metal surfaces. Because of this growth process, the nanowires grow radial direction before covering the entire surface. The nanowire surface is one of the promising geometry for superhydrophobicity. In fact, anodic nanowire film surface becomes superhydrophobic after fluoroalkylsilane monolayer coating.
The anodic nanowire film is also developed on copper by anodizing in KOH electrolyte. The copper surface can be completely covered with the nanowires only for 30 s at a constant current density of 10 mA cm–2. The nanowires consist of a single crystalline Cu(OH)2 phase, and the nanowires on copper are formed uniformly without forming pits. The nanowires are reduced readily to copper metal with the nanowire morphology remaining almost unchanged, although the copper metal nanowires are polycrystalline. The copper metal nanowires are also superhydrophobic after fluoroalkylthiol monolayer coating. The copper substrate with the superhydrophobic metal nanowires maintain the high thermal conductivity and effectively removes the condensed water droplet, being very suitable for heat exchanger applications with high efficiency.
Aluminum is the most suitable metal to control the surface morphology by wet process. The author’s group successfully fabricated hierarchically micro-/nano-porous aluminum surface by a combination of chemical etching and anodizing in acid electrolytes. Micropits are developed by chemical etching containing HCl solution and the HCl concentration control the size of pits. Nanopores are generated by anodizing in sulfuric acid or oxalic acid, the pore widening process can be applied to control the pore size and porosity. The hierarchically porous aluminum surface is used to form super-repellency even for low surface tension liquids, including rapeseed oil (surface tension of 35 mN m–1), hexadecane (28 mN m–1), octane (22 mN m–1), and even hexane (18 mN m–1). The nanopores can be used to infiltrate the self-healing agent of organic coating, so that self-healing super-liquid-repellency occurs repeatedly even if the thin organic coating is damaged. The porous aluminum surface is also suitable for fabricating the slippery liquid infused porous surface (SLIPS) by impregnating a lubricant liquid into the pores. The SLIPS on aluminum show excellent liquid slipping property, anti-snow-sticking property, and high corrosion resistance.
References

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