Morphology and phase control of iron oxide polymorph nanoparticles

In this work, lepidocrocite ( γ -FeOOH) nanobundles were prepared by a facile NH₄F assisted epoxide precipitation route. The reactions between epoxide and [Fe(H₂O)₆]²⁺ promoted the hydrolysis and condensation of [Fe(H₂O)₆]²⁺, resulting in the formation of iron oxyhydroxide. After calcination of γ -FeOOH nanobundles at 400 °C, the produced α -Fe₂O₃ still kept the bundle morphology. Due to the unique chemistry of epoxide, the morphology and phase of iron oxide polymorph nanoparticles (goethite, akaganeite, lepidocrocite, magnetite) were well-controlled through controlling reaction conditions such as Fe²⁺ concentration, NH₄F additive and reaction temperature. It is particularly interesting that NH₄F working as phase controlling agent is able to control the phase development of iron oxyhydroxides. This phase control effect of NH₄F is attributed to the promoted reaction rate of epoxide originating from the higher electronegativity of fluoride ions than chloride ions. Based on the results in this work and our other preliminary works, it is considered that this route can be used as a general strategy for controlling the morphology and phase of transition element compounds. (paper)