Hollowing of MnO Nanocrystals Triggered by Metal Cation Replacement: Implications for the Electrocatalytic Oxygen Evolution Reaction

Manganese oxide (MnOx) based hollow nanocrystals (NCs) represent a promising class of materials for catalysis. The conventional routes to synthesize such nanostructures rely on the use of hard/soft sacrificial templates or on high reaction temperatures. Herein, we report a template-free method to rapidly transform nonhollow MnO NCs into hollow nanostructures at room temperature. Our synthesis method is based on the reaction of guest metal cations Mδ+ (e.g., Fe2+, Ce3+, Fe3+, etc.) with MnO NCs, the latter being actually passivated by a thin Mn3O4 layer. The guest cations replace part of the Mn ions in the Mn3O4 shell up to a critical threshold value (e.g., ∼12 at. % when working with Fe2+ guest cations), above which etching of the MnO core rapidly occurs. Our analyses suggested that the etching of the core could be related to the release of strain that is built up between the MnO core and the Mn3-xMxO4 shell as the exchange progresses. When Fe2+ ions are employed as guest cations, the resulting Mn3–xFexO4 hollow NCs exhibit high activity in the electrocatalytic oxygen evolution reaction due to their large electrochemical surface area and low charge transfer resistance. Our work provides an easy and green synthesis strategy to prepare hollow NCs with control over the composition and size, which are features that make them efficient electrocatalysts.