Role of the Counteranions on the Formation of Different Crystal Structures of Iron Oxyhydroxides via Redox Reaction

Iron-based oxides are attractive nanomaterials due to their high stability, low cost, low toxicity, and abundance that have been used in a wide range of applications such as superconductors, pigments, catalysts, magnetic materials, and sensors. Iron oxyhydroxides are a particularly interesting type of iron oxides due to their enhanced capacity to extract metal ions from solution by adsorption of these ions to hydroxide groups on the surface. Various methods have been reported for the synthesis of iron oxyhydroxides nanostructures such as hydrolysis of iron salt, oxidation of iron sulfide by H₂O₂/H₂O, and microwave assisted method. Conversion chemistry is another approach for the synthesis of nanomaterials, where a certain type of nanostructure serves as template and is converted into a new type of nanostructure with different composition, while morphological changes can occur.

Redox reactions have been employed for the conversion of various metal oxide nanocrystals, where several parameters were examined to understand the reaction mechanism. However, the role of the counteranions still has not yet been studied. Herein, we present the influence of the counteranions (sulfate, chloride, and acetate) on the formation of iron oxyhydroxide nanofiber structure (100 nm × few microns; diameter × length) via redox reaction using iron salt and manganese oxide as a template. We found that different structures of iron oxide/oxyhydroxide (goethite, hydrohematite, and maghemite) were obtained when different anions (sulfate, chloride, and acetate, respectively) were used. Moreover, we studied the effect of various parameters (such as the concentration of iron precursor, the time, and the temperature of the reaction) on the morphology, crystallinity, and composition of the final products. Finally, the iron oxyhydroxide nanofibers was further converted to hematite while preserving the original morphology of the converted nanostructures. This can further expand the application of these unique nanostructures to various fields such as water-splitting