Atomic Reconstruction and Oxygen Evolution Reaction of Mn₃O₄ Nanoparticles

Abstract

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 Mn 3 O 4 nanoparticles. We image surface structures of Mn 3 O 4 nanoparticles and observe surface reconstructions in the ( 110) and ( 101) planes. Mn 3+ ions at the surface, which are commonly considered as the active sites in OER, disappear from the reconstructed planes, whereas Mn 3+ 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.S urface chemistry and reactions are fundamental to the properties of various functional materials, including solid catalysts, supercapacitors, and sensors. 1 At the molecular level, surface atoms interact differently with surrounding molecules depending on their local atomic arrangements, such as facet, step, edge, and corner atoms, and surface defects, causing considerable variations in the chemical environment. 2 Furthermore, surface relaxation and reconstruction also significantly alter chemical reactions at the surface because of modified electronic structures. Together, surface atomic arrangements and surface reconstructions constitute what is broadly defined as surface structures. Because of the complexity of surface structures, atomic-level characterization is essential for a molecular-level understanding of surface chemical reactions. For example, identification of the active sites among various surface atoms in catalysis can lead to direct advances in catalyst design. 3 While a variety of advanced surface characterization techniques have been developed and employed to obtain information associated with chemical reactions at specific surface sites, most techniques have their own restrictions for the surface types that can be studied, and accurate measurement of atomic and electronic structures with high precision remains difficult in nanostructured materials. 4 Surface profile imaging 5,6 by transmission electron microscopy (TEM), particularly scanning transmission electron microscopy (STEM) in combination with electron energy loss spectroscopy (EELS), is a novel technique for the analysis of nanoparticle surfaces. The high-angle annular dark-field (HAADF) mode in STEM allows individual atomic columns to be located, and EELS provides information on the electronic and chemical states simultaneously. Recently, the capability of this technique has improved significantly due to the dramatic advances in aberration correction. Surfaces of the ...

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