Figure 1 (a) Schematic illustration of synergistic surface optimization and proposed catalysis function. (b) HRTEM image of Pt-SMO-Co2N NWs. (c) HADDF image of Pt-SMO-Co2N NWs. (d) Co 2p XPS pattern of Co2N NWs, Pt-SMO-Co2N NWs and Ar-Pt-SMO-Co2N NWs. (e) Pt 4f XPS pattern of Pt-SMO-Co2N NWs and Ar-Pt-SMO-Co2N NWs.
To further investigate the local structure of cobalt and platinum in above catalysts, in-situ X-ray absorption fine structure (XAFS) was then employed to monitor the structure evolution during surface microenvironment optimization. Both Co K-edge and Pt L3-edge spectra were recorded for the pristine electrocatalyst, and the electrocatalysts after surface microenvironment optimization for 40 min, 80 min and 120 min. As shown in Figure 2a , despite a slight right shift toward higher absorption energy, the Co K-edge XANES spectra of the Pt-SMO-Co2N NWs were still similar to pristine Co2N NWs, suggesting that the bulk lattice framework was maintained. From the magnified spectra, a continuous right shift could be observed as the time increased from 0 to 120 min, indicating the surface was gradually oxidized to higher valence during the electrochemical surface optimization. The Fourier transformed spectra of Co K-edge were shown in Figure 2b . During surface microenvironment optimization, EXAFS spectrums underwent a continuous decrease on Co-N bonds, while an increase of Co-O bonds with the elongated electrochemical treatment, indicating the formation of oxyhydroxide layer26. In-situ XAFS was also operated to investigate local structure of Pt during surface microenvironment optimization at different stages (Figure 2c and 2d) . The peak at 2.41Å could be ascribed to Pt-Pt bonds in Pt-SMO-Co2N NWs, of which intensity increased with extended electrochemical oxidation duration. To illustrate the evolution of Pt local structure more straightforwardly, the wavelet transform (WT) analysis was performed (Figure 2e and 2f) 28. The WT contour plots also showed the same trend as that of Fourier transformed Pt L-edge spectra. All these results clearly indicate the formation of sub-nano Pt cluster deposited on generated oxyhydroxide layer on the surface of Co2N NWs.