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).