3.2 Electronic structure of Ni atoms in Ni@N-C SAC
The XPS was used to characterized the chemical composition and valence state of Ni@N-C SAC (Figure S2). The binding energy of the Ni 2p3/2 peak is 855.1 eV, which is higher than that reported for Ni0 (853.4 eV) and lower than that for Ni2+ (855.7 eV) 43, 44. The high-resolution XPS N 1s spectrum can be deconvoluted into pyridinic N (∼398.1 eV), Ni-N (∼399.2 eV), pyrrolic N (∼400.4 eV), graphite N (∼401.3 eV) and N-oxidized (∼403.0 eV)45. Based on the XPS data (Table S1), the atomic concentration of Ni in Ni@N-C SAC is 8.01 wt%.
To further study the electronic structures of Ni species in Ni@N-C SAC, the analysis of XANES and EXAFS was conducted. Figure 2a shows the Ni K-edge XANES spectra of Ni@N-C SAC and two references: Ni foil and NiO. In the first derivative XANES spectra, the intensity lines (absorption edges) of Ni@N-C SAC lie between those of Ni foil and NiO, demonstrating the unique electronic structure of Niδ+(0<δ<2) in Ni@N-C SAC, where the valence of the Ni species is between Ni0 and Ni2+. This observation is consistent with the aforementioned XPS results. The Fourier transform of the k3-weighted Ni K-edge EXAFS spectrum of Ni@N-C SAC exhibits a dominant peak at 1.41 Å, corresponding to the Ni-N coordination, which is shorter than both the Ni-O peak in the NiO spectrum and the Ni-Ni peak in the Ni foil spectrum (Figure 2b). The wavelet transform was also employed to assess the Ni K-edge EXAFS oscillations of Ni@N-C SAC. As shown in Figure 2c, the wavelet transform analysis of Ni@N-C SAC displays only one intensity maximum at 5.0 Å-1, which is ascribed to the Ni-N coordination. The above results further prove the presence of atomically dispersed Ni in Ni@N-C SAC. The EXAFS fitting analysis was applied to investigate the configuration of Ni atoms in Ni@N-C SAC. The calculated coordination number of Ni is ca 4 and the mean bond length of Ni@N-C SAC is 1.88 Å (Table S2). Hence, each Ni atom is confined in the N-doped carbon matrix by coordinating with four N (Ni-N4 structure, Figure S3).