Synthesis of hierarchical structured Fe₂O₃ rod clusters with numerous empty nanovoids: Via the Kirkendall effect and their electrochemical properties for lithium-ion storage

The Kirkendall effect has been widely applied to prepare hollow/porous metal-oxide-based composites. We propose a new mechanism for the transformation of hierarchical structured metal selenides into their corresponding metal oxides with unique structures via the Kirkendall effect. Based on this mechanism, hierarchical structured iron oxide clusters comprising one-dimensional nanorods with numerous empty nanovoids were successfully prepared. FeSe₂ rod clusters synthesized by a one-pot hydrothermal process were post-treated under an air atmosphere. During the oxidation process, FeSe₂@FeO_x-Se@Fe₂O₃ and FeO_x-Se@Fe₂O₃ intermediates are formed owing to the different diffusion rates of iron cations, the selenium component, and oxygen gas. As the oxidation proceeded, elimination by evaporation of the SeO₂ layer formed by the reaction of the diffused-out metalloid Se and oxygen gas and oxidation of FeO_x resulted in porous Fe₂O₃ nanorods with numerous interconnected empty nanovoids. When employed as a lithium-ion battery anode, hierarchical Fe₂O₃ rod clusters exhibited high reversible discharge capacity, good cycling stability, and excellent rate performance. Following 200 cycles, their discharge capacity is 1318 mA h g^-1 at a current density of 1 A g^-1. Additionally, the rod clusters delivered a discharge capacity of 745 mA h g^-1 at a high current density of 10 A g^-1.