Compositional variations in anodic nanotubes/nanopores formed on Fe 100, 110 and 111 single crystals

Abstract

Anodizing of iron (100), (110) and (111) single crystals in mono-ethylene-glycol electrolyte containing 1.5 mol dm−3 water and 0.1 mol dm−3 ammonium fluoride leads to formation of anodic iron nanopores/nanotubes where the nanopores are essentially composed of oxide nanotubes separated by iron fluoride matrix. It was found that electrochemical thermodynamics for nanoporous/nanotubular film formation apparently depends on the index number of facet on which the anodic film is formed. The film formation on (100) facet is associated with extended gas evolution upon anodizing and consequently corresponds to a shift of current-time curve towards higher current density values comparing with those formed on higher index number facets. The nanotubes formed on (100) facet have a general chemical formula of Fe2O3.FeF2, whereas those formed on higher index number are composed of Fe3O4.FeF2. The anodic films formed on Fe (110) and Fe (111) are essentially amorphous whereas the one formed on (100) facet shows high degree of crystallinity. The results are discussed in view of anisotropic properties of iron.