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.