Oxygen Atom Migration Dominates Anomalous Reversible Oxidation of Ru Nanosheets

Abstract Oxidation is a ubiquitous reaction in nature. For most metals (especially metal nanoparticles [NPs]), they will be completely oxidized under suitable conditions, except ruthenium (Ru). In this work, in situ transmission electron microscopy (in situ TEM) and ex situ spectroscopy were employed to investigate the oxidation process of Ru nanosheets. Ex situ spectroscopic analysis demonstrates the incomplete oxidation of Ru nanosheets to RuO₂, while the in situ TEM observations uncover an anomalous reverse phase transformation from the oxidized to metallic phase during oxidation. Combined with theoretical calculations, the oxygen atom migration dominates the reversible oxidation process, strikingly distinct from the unidirectional oxidation pathways in conventional metallic systems. The as‐generated abundant Ru─RuO 2 heterointerfaces formed through reversible oxidation provide a wealth of active sites for electrochemical alkaline hydrogen evolution reaction (HER). Herein, the study not only lays a foundation for the understanding complex dynamic oxidation processes, but also offers new insights into the design of nanocatalysts.