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.