Formation of Nanoporous NiFe2O4 Oxide Films By Anodizing of Fe-Ni Alloy

Abstract

Anodizing of metals and alloys has attracted increased attention for the formation of nanostructured oxide films with self-ordered morphologies. The anodizing researches have been conducted mainly on valve metals, including aluminum, magnesium, titanium, zirconium, niobium and tantalum, because of ready formation of thick anodic films on these metals. Recent findings of organic electrolytes containing fluoride have expanded the anodizing studies to non-valve metals; nanoporous and nanotubular anodic films are now available even on iron and stainless steel 1-3. The nanoporous and nanotubular α-Fe2O3 films have been formed by anodizing of iron and subsequent heat treatments, and such oxide films are of interest for photoelectrochemical water splitting 4. Further functional oxide films will be formed by anodizing of iron alloys. In the present study, we examined the formation of nanoporous anodic films on an Fe-Ni alloy. The oxygen evolution properties of the anodic oxide films were also examined. An Fe-Ni invar alloy plate containing 41 mass% nickel was used in this study. For comparison, a high purity (99.99%) iron plate was also used. The specimens were anodized at 40 V in ethylene glycol electrolyte containing 0.1 mol dm-3 NH4F and 0.5 mol dm-3 H2O at 293 K. For anodizing of iron, the H2O concentration was increased to 1.5 mol dm-3 because the detachment of an oxide film formed at the H2O concentration of 0.5 mol dm-3 proceeded during heat treatment. After anodizing, heat treatment of the specimens was conducted in an argon atmosphere at temperatures between 673 K and 873 K to remove fluoride species in the anodic films and increase the crystallinity of the oxide films. During anodizing of the Fe-Ni alloy at 40 V the current density exceeded 200 A m-2, which was approximately 4 times that of iron in the same electrolyte. Because of the high current density, the film growth rate on the Fe-Ni alloy was as high as 0.3 μm min-1 and a 3-μm-thick anodic film was formed by anodizing only for 10 min. The anodic film exhibited a cylindrical nanoporous morphology, which was similar to that formed on iron. However, the pore size and the interpore distance for the Fe-Ni alloy were smaller than those on iron. The as-anodized specimens contained fluoride species in the anodic films, as confirmed by glow discharge optical emission spectroscopy depth profile analysis. The heat treatment of the anodized alloy in argon atmosphere at 873 K removed fluoride species and a crystalline spinel phase was developed. When anodized iron specimens were heat-treated in the argon atmosphere at 673 and 773 K, α-Fe2O3 phase was also detected by XRD as well as the Fe3O4 phase. The a-Fe2O3phase disappeared at 873 K. The oxygen evolution reaction (OER) was examined in 0.1 mol dm-3 KOH solution by anodic polarization measurements. For the iron specimens, the OER activity increased with an increase in the heat treatment temperature from 773 K to 873 K. The presence of α-Fe2O3heat-treated at 773 K may be associated with the lower OER activity. The alloy specimen heat-treated at 873 K showed further enhanced activity, confirming the beneficial effect of nickel incorporation into the anodic oxide for enhancing the OER activity.