Morphological evolution of fe-oxy-hydroxide nanotubes during electrodeposition

Background: Ordered arrays of 1D iron(oxyhydr)oxide nanostructures have potential applications in magnetic recording mediums, lithium batteries, supercapacitors, and thermal production of α-, β-, γ-type Fe2O3. Large surface areas with three-dimensional architectures, such as nanotubes, are encouraged because the easy access of ion, gas, liquid and radiation assures high ion exchange capacity, sensing and catalytic activities. Objective: In this work, the morphological evolution of Fe-oxyhydroxide electrodeposition inside AAM pores has been followed for the first time by selecting two relevant electrochemical conditions of synthesis producing high quality morphologies of nanotubes. Methods: Iron(oxyhydr)oxide nanotubes have been synthesized by cathodic electrodeposition at a constant current in classic three-electrode cell. Two different electrolytic baths have been studied: (i) an aqueous bath consisting of 5 mM FeCl3+5 mM KF+0.1 M KCl+1 M H2O2 (H-Fe) and (ii) an ethanolic bath consisting of 0.3 M FeCl3 + 0.1 M KCl (Et-Fe). Results: XRD, Raman and SEM results on the iron(oxyhydr)oxide nanotubes suggest different mechanisms of chemical precipitation mechanisms in Et-Fe alcoholic solution (dehydration and rearrangement within the ferrihydrite aggregates) and H-Fe aqueous solution (dissolu-tion/reprecipitation). The morphological evolution of the growing nanostructure to nanotubes inside AAM in the two baths agrees very well with the overpotential vs. time curves, the kinetic growth of the nanotubes arrays and a growth mechanism governed by the relative mass transfer processes involving both OH- and Fe ions. Conclusion: The morphological evolution of Fe-oxyhydroxide cathodic electroprecipitation inside AAM pores in two relevant electrochemical baths containing Fe(III) (aqueous/H-Fe and alcoholic/Et-Fe) has been followed for the first time by a comprehensive SEM analysis accompanied by electrochemical, structural and kinetic growth of the nano-electrodeposits. The detailed SEM results collected in this work allowed to recommend template electrogeneration of base in ethanol solution containing Fe(III) chloride as a relevant procedure to obtain high-quality, compact and well-ordered Fe oxy-hydroxide nanotubes.