Figure 3. a) XRD patterns of ACP/RuSAC+C and ACP/RuC. b) Magnified zone of the (002) reflection. c) H2-TPD measurements of ACP/RuSAC+C and bare CP. d, e) AC HAADF-STEM image of ACP/RuSAC+C. f) Statistical distance between Ru single atoms and the surface of Ru nanoclusters in AC HAADF-STEM images. g) Intensity profiles obtained in area 1 to 4 in Figure e). h) HAADF-STEM image and corresponding element mapping images of C, N, and Ru elements (red, blue, and pink colors represent C, N, and Ru, respectively).
XPS was conducted to analyze the compositions and chemical states of the electrodes. The overall spectrum of ACP/RuSAC+C is depicted in Figure S4, Supporting Information, from which the elements Ru, N, O, and C can be observed. In the Ru 3p spectrum, peaks corresponding to both Ru0 (462.2 eV, 484.4 eV) and Run+ (464 eV, 487.7 eV) were observed for ACP/RuSAC+C (Figure 4 a). Additionally, the N 1s spectrum in Figure 4b provides evidence of the presence of a Ru-N bond at 399 eV, further confirming the presence of both single atoms and nanoclusters in ACP/RuSAC+C. For the ACP/RuC, the analysis revealed two peaks corresponding to Ru0 at 462.08 and 484.2 eV, as depicted in Figure S5, Supporting Information. A comparative analysis of the Ru 3p spectra of the two electrocatalysts in Figure 4c demonstrates a positive shift in the binding energy of the metallic Ru species on ACP/RuSAC+C by approximately ~0.2 eV. This shift suggests electron transfer from Ru clusters to Ru single atoms, due to the electronegativity of the Ru-N bond in the Ru single atoms.[14,32] This synergistic effect between the Ru nanoclusters and Ru single atoms of ACP/RuSAC+Ccontributes to its superior performance in the HER, and this will be further explored through theoretical density functional theory (DFT) calculations. Synchrotron-based X-ray absorption spectroscopy (XAS) was conducted to investigate the electronic configurations and atomic coordination environments of the samples. Figure 4d presents the Ru K-edge X-ray absorption near-edge structure (XANES) spectra of ACP/RuSAC+C. The absorption edge of ACP/RuSAC+C lies between those of the Ru foil and RuO2, indicating that the chemical valence of Ru is between 0 and +4. As observed from the corresponding Ru K-edge Fourier-transformed extended X-ray absorption fine structure (FT-EXAFS) spectra of the ACP/RuSAC+C in Figure 4e, two prominent peaks were identified at approximately 1.56 and 2.53 Å, which belong to the Ru-N, and Ru-Ru coordination, respectively. Additionally, it can be obtained from the EXAFS fitting in the R-space that the coordination number of Ru-N is 4 (Figure S6, Supporting Information). The wavelet transform (WT) of EXAFS in k-space was further employed to confirm the coordination information on the Ru atoms (Figure 4f). The WT analysis of ACP/RuSAC+C showed two prominent maximum values at 4.0 and ~8 Å–1, where the former peak corresponds to Ru-N coordination, while the latter peak corresponds to Ru-Ru bond. In addition, compared with the WT signals of ACP/RuSAC+C, only Ru-Ru coordination was observed in ACP/RuC.