Impact of water and oxidation states in the galvanic replacement formation of hollow oxide nanoparticles

Abstract

Metal‐oxide hollow nanoparticles are appealing structures from the applied and fundamental viewpoints. The synthesis of bi‐phase metal‐oxide hollow nanoparticles has been reported based on galvanic replacement using an organic‐based seeded‐growth approach, but with the presence of H2O. Here we report on a novel route to synthesize hollow core‐shell MnOx/FeOx nanoparticles by galvanic replacement without the use of H2O. We demonstrate that the role of H2O in the MnOx/FeOx galvanic replacement synthesis is to oxidize the MnO/Mn3O4 seeds into pure Mn3O4 in order to obtain the suitable oxidation state so that the Mn3+→Mn2+ reduction by the Fe2+ ions can occur. Thus, if no H2O is added, onion‐like MnO/Mn3O4/Fe3O4 nanoparticles are obtained, while whenMn3O4 seeds are used, hollow core‐shell MnOx/FeOx are achieved. Thus, a critical step for galvanic replacement is the existence of proper oxidation states in the seeds so that the chemical reduction by the shell ions is thermodynamically favoured.