Results and discussion
To realize the synergetic optimization of intrinsic activity and microenvironment for active sites, electrochemical oxidation was conducted by applying a voltage from 0V to 1.6V on Co2N nanowires in a Pt precursor contained solution (Figure 1a) . To follow the surface evolution of Co2N nanowires, HRTEM was operated to study the surface microstructure of Co2N before and after surface microenvironment optimization. Figure S3 shows the HRTEM image of pristine Co2N nanowires with a lattice spacing value of 0.196 nm which could be ascribed to (120) plane. After the microenvironment optimization, a rough surface layer with small Pt clusters could be clearly observed in Figure 1b . High-angle annular dark-field scanning transmission electron microscopy (HAADF) further confirmed the formation of sub-size Pt clusters with a diameter of 1~2 nm (the bright areas inFigure 1c ). The in-situ X-ray photoelectron spectra (XPS) equipped with Ar ions etching was operated to investigate the composition and electronic structure of the oxidized surface layer as a function of depth. As shown in Figure 1d , the fine-scanned Co 2p XPS spectra of the pristine Co2N nanowires and the Pt-SMO-Co2N NWs can be well fitted with three different valence states (i.e. Co3+ at 795.2 eV, Co2+ at 797.0 eV, and Co0 at 794.5 eV)25,26 The filled area under each fitting curves represents the abundance of each valence state. The increased surface Co3+/Co0 ratio after surface microenvironment optimization suggests the formation of more Co3+ species through the oxidation of surface Co0 of Co2N nanowires, which is probably corresponding to high valence CoOOH layer according to previous studies26. With 3 keV Ar etching, the ratio of Co3+/Co0 was decreased to nearly the same level with pristine Co2N nanowires, indicating that the internal phase of Co2N was maintained. Moreover, as shown in Figure 1e , two fitted peaks at 70.8 eV and 74.1 eV could be ascribed as the Pt 4f5/2 and 4f7/2 bands of Pt027. With 3 keV Ar etching of surface layer, the peak intensity of Pt 4f5/2 and 4f7/2 was greatly reduced, suggesting that sub-nano Pt sites were deposited on the most surface layer after microenvironment optimization.