Charge-transfer modified embedded-atom method for manganese oxides: Nanostructuring effects on MnO2 nanorods

Abstract

In spite of the superior performance of nanostructured manganese oxides as materials for electro-catalysis, there has been low progress in their theoretical understanding because of the large scale of the required simulation cells. In this work, we have developed a large-scale simulation method for manganese oxides based on the Charge-Transfer Modified Embedded-Atom Method (CT-MEAM), which is not only capable of describing complex atomic bonding in transition metal oxides but also dynamic oxidation states, as those induced by the widely exposed surface area of nanostructures. The reliability of the method is confirmed by comparison with the corresponding ab initio reference results, and afterwards used to investigate the nanostructuring effects on manganese oxide nanorods. We found that nanostructuring introduces surface and edge structural distortions, driven by emerged unsaturated surface ions. These ions are highly mobile, and easily migrate to form new bonds with neighboring ions. Nanostructuring also weakens charge transfer between Mn and O ions located at the outer layers, thus bringing in more reductive Mn ions and enabling oxygen vacancy formation, which ultimately enhances the catalytic activity of manganese oxides. Our results also show that large scale surface defects, such as voids, could produce a similar effect than size reduction in improving the catalytic performance of the nanorod, thus avoiding undesirable amorphization instabilities, which are inevitable at reduced sizes. These findings will help to the optimization of the design and synthesis of nanostructured manganese oxides with enhanced catalytic activity.