Effects of anodization conditions of stainless steel on the formation of ordered nanoporous structures with high aspect ratios

Abstract The nanoporous structures obtained by the anodization of stainless steel are functional materials with various potential applications. It has been reported that nanoporous structures can be prepared by the anodization of stainless steel in an electrolyte containing fluoride ions. However, under the reported anodization conditions, the control range of the interpore distance of resulting nanoporous structures was narrow. To expand the application fields of the nanoporous structures obtained by the anodization of stainless steel, it is an important challenge to determine the anodization conditions that can control the interpore distance of nanoporous structures over a wide range. In this study, we investigated the effects of the electrolyte composition on the anodization behavior of stainless steel and the interpore distance of the resulting nanoporous structure. As a result, we found that the maximum voltage for the stable anodization of stainless steel increases when a mixture of ethylene glycol and glycerol containing NH4F is used as the electrolyte. Since the interpore distance of nanoporous structures obtained by the anodization of stainless steel is proportional to the anodization voltage, as the voltage range over which stainless steel can be anodized increased, the range of interpore distances of the nanoporous structures obtained also increased. On the basis of these results, ordered nanoporous structures with a large interpore distance (100 nm), which could not be obtained under the previously reported anodization conditions, were fabricated by the anodization of a stainless steel substrate with a depression pattern formed by Ar ion milling using an alumina mask under optimized anodization conditions. The resulting ordered nanoporous structures with controlled interpore distances are expected to be used in various devices such as capacitors and photocatalysts.