Oriented α-Fe2O3 and Fe3O4 nanoporous films obtained by topotactic-like pseudomorphic transformation of γ- and δ-FeOOH films

Abstract

Nanoporous metal oxide films comprising nanocrystal assemblies with an aligned crystallographic orientation are the key nanostructures that provide efficient interfacial reaction sites. However, there is a lack of simple, template-free methods for synthesizing such films. Herein, we report the facile formation of oriented nanoporous hematite (α-Fe₂O₃) and magnetite (Fe₃O₄) films, through a topotactic-like pseudomorphic transformation of iron oxyhydroxide (FeOOH) films. Two types of oriented FeOOH films, [010]-γ-FeOOH and [001]-δ-FeOOH, were formed on substrates through the electrodeposition of layered iron hydroxide (green rust) films in an Fe²⁺–NO₃⁻ aqueous solution and subsequent drying in air. Furthermore, by subjecting these oriented FeOOH films to heat treatment in air and vacuum, oriented α-Fe₂O₃ and Fe₃O₄ nanoporous films were obtained, with different preferred growth orientations: [010]-γ-FeOOH was transformed into (104)- and (116)-α-Fe₂O₃ and [110]-Fe₃O₄; [001]-δ-FeOOH was transformed into [001]-α-Fe₂O₃ and [111]-Fe₃O₄. In addition to the structural, morphological and chemical characterizations, optical and magnetic properties of the obtained films were examined. A plausible explanation for the observed orientation relationships between the precursors (γ- and δ-FeOOH) and the products (α-Fe₂O₃ and Fe₃O₄) is given based on the pyrolysis intermediates and the similarity in their atomic arrangements.