3.1 Chelation-anchored strategy for constructing Ni@N-C SAC
The Ni@N-C SAC was prepared by the mean of a two-stage pyrolysis of a mixture of D-glucosamine hydrochloride, nickel acetate and melamine, which were individually served as chelating agent, metal precursor and soft-template, respectively (Figure 1a). In the first stage (<600 ºC), the thermal condensation of melamine produced graphite carbon nitride (g-C3N4), and thermal degradation of D-glucosamine hydrochloride formed a carbon skeleton between g-C3N4layers41. In the second stage, g-C3N4 began thermolysis at a temperature above 700 ºC and finally formed N-doped graphene-like nanosheets, providing a nitrogen source for the formation of Ni-Nx structure when the temperature reached 800 ºC. The morphology of Ni@N-C SAC was characterized by SEM and TEM.
As shown in Figure 1b and 1c, Ni@N-C SAC is composed of wrinkled graphene-like nanosheets. The TEM image of Ni@N-C SAC only shows one atom lattice structure (d=3.52 Å), which is attributed to graphite carbon (Figure 1d). The XRD spectrum of the Ni@N-C SAC has two broad peaks at 26.3º and 41.2º, which are corresponding to 002 peak and 100 peak of graphitic carbon42, while diffraction peaks of Ni are not detected (Figure S1). However, aberration-corrected high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) shows that Ni single atoms are homogeneously distributed across the wrinkled graphene-like nanosheets (Figure 1e-f). And the average diameter of the spots is 1.67 ± 0.29 Å based on the statistical analysis of over 100 bright spots (the top right corner of Figure 1f). In addition, the EDS-mapping analysis reveals the homogeneous distribution of Ni, N and C atoms across the whole graphene-like nanosheets (Figure 1g). These above results indicate that Ni is atomically distributed on the carrier.