Monitoring Dynamics of Defects and Single Fe Atoms in N-functionalized Few-Layer Graphene by in Situ Temperature Programmed Scanning Transmission Electron Microscopy

In this study,we aim to contribute an understanding of the pathway of formation of Fe species during top-down synthesis of dispersed Fe on N-functionalized few layer graphene,widely used in electrocatalysis.We use X-ray absorption spectroscopy to determine the electronic structure and coordination geometry of the Fe species and in situ high angle annular dark field scanning transmission electron microscopy combined with atomic resolved electron energy loss spectroscopy to localize these,identify their chemical configuration and monitor their dynamics during thermal annealing.We show the high mobility of peripheral Fe atoms,first diffusing rapidly at the trims of the graphene layers and at temperatures as high as 573 K,diffusing from the edge planes towards in-plane locations of the graphene layers forming three-,four-coordinated metal sites and more complexes polynuclear Fe species.This process occurs via bond C-C breaking which partially reduces the extension of the graphene domains.However,the vast majority of Fe is segregated as a metal phase.This dynamic interconversion depends on the structural details of the surrounding graphitic environment in which these are formed as well as the Fe loading.N species appear stabilizing isolated and polynuclear Fe species even at temperatures as high as 873 K.The significance of our results lies on the fact that single Fe atoms in graphene are highly mobile and therefore a structural description of the electroactive sites as such is insufficient and more complex species might be more relevant,especially in the case of multielectron transfer reactions.Here we provide the experimental evidence of the formation of these polynuclear Fe-N sites and their structural characteristics.