Atomic Reconstruction and Oxygen Evolution Reaction of Mn 3 O 4 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 MnO nanoparticles. We image surface structures of MnO nanoparticles and observe surface reconstructions in the (110) and (101) planes. Mn ions at the surface, which are commonly considered as the active sites in OER, disappear from the reconstructed planes, whereas Mn 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.