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.