Atomic Reconstruction and Oxygen Evolution Reaction of Mn₃O₄ Nanoparticles

Understanding the chemical states of individual surface atoms and their arrangements is essential for addressing several current issues such as catalysis, energy stroage/conversion, and environmental protection. Here, we exploit a profile imaging technique to understand the correlation between surface atomic structures and the oxygen evolution reaction (OER) in Mn₃O₄ nanoparticles. We image surface structures of Mn₃O₄ nanoparticles and observe surface reconstructions in the (110) and (101) planes. Mn³⁺ ions at the surface, which are commonly considered as the active sites in OER, disappear from the reconstructed planes, whereas Mn³⁺ ions are still exposed at the edges of nanoparticles. Our observations suggest that surface reconstructions can deactivate low-index surfaces of Mn oxides in OER. These structural and chemical observations are further validated by density functional theory calculations. This work shows why atomic-scale characterization of surface structures is crucial for a molecular-level understanding of a chemical reaction in oxide nanoparticles.