Fig. 1. (a) Synthesis route of CoFe-CoxN@NC
heterojunction, XRD pattern of CoFe-CoxN@NC with various
(b) metal ratios, (c) pyrolysis temperatures and (d) urea dosages
The
delicate morphology of CoFe-CoxN@NC was investigated by
SEM, TEM and HRTEM, as shown in Fig. 2. Compared with the metal free
sample NC, the pore structure of CoFe-CoxN@NC exposed
more active sites and the metal particles were highly dispersed without
agglomeration, thus would improve the catalytic activity. In addition,
HRTEM images of CoFe-CoxN@NC in Fig. 3d-f showed the
clear lattice of CoFe alloy nanoparticles and the lattice spacing was
0.202 nm, which can be ascribed to the (110) diffraction plane of CoFe
alloy; a distinct lattice fringe of 0.207 nm, consistent with the (111)
lattice plane of Co5.47N; the lattice spacing of the
outer carbon layer is about 0.34 nm, attributing to the d -spacing
of the (002) plane in graphitic carbon. Moreover, the energy dispersive
X-ray spectrometer (EDX) showed that Co, Fe, C, N and O elements were
uniformly distributed along the surface of CoFe-CoxN@NC
(Fig. 2g). CoFe-CoxN heterojunction particles with sizes
ranging from 20.2 nm to 58.6 nm were encapsulated by the graphitic
carbon layer and uniformly distributed on the carbon support. The
SEM-EDX image of CoFe-Co5.47N@NC was shown in Fig. S5a,
and the average particle size was 31.36 nm and the particle size
distribution was shown in Fig. S5b. The results above proved the
formation of CoFe alloy and CoxN heterojunction
encapsulated by the graphic carbon layers and located on the
nitrogen-doped lignin-derived shell-core carbon support.