## 4 Results and Discussion  
Aberration-corrected scanning transmission electron microscopy (AC-STEM) was utilized for revealing the surface morphology and crystal structure of experimental NCs. Figure 1 represents the comparative STEM images of Ni@Pd and Ni@Pd-Pt NCs. Accordingly, Ni@Pd (Figure 1a) nanoparticles exhibit a hierarchical structure, where long-range ordered atomic arrangements are observed on the surface due to capping of Pd-atoms over disorders Ni-atoms underneath. Such a characteristic confirms the formation of the hierarchical structure of Ni@Pd NC with a high density of surface defects (yellow arrows denote surface defects in Figure 1a), in which Pd-atoms are stacked over Ni-atoms. For the Ni@Pd-Pt catalyst, sub-nanometer to nanometer-scaled decorated Pt-clusters (indicated by the red arrow in Figure 1b) were observed on the Ni@Pd surface exhibiting the highest Z contrast in the HAADF-STEM image. Meanwhile, due to its highest Z number (Z = 78) in the ternary system they could be determined as Pt. Figure 1: Aberration-corrected scanning transmission electron microscopy (AC-STEM) images of as-prepared (**a**) Ni@Pd and (**b**) Ni@Pd-Pt nanocatalysts (NCs). Further structural information of the experimental NCs under investigation was obtained via XRD analysis. Figure 2 depicts the XRD spectra of Ni@Pd-Pd NC compared with control samples (Ni@Pd, Pd/C and Pt/C). Accordingly, Pt/C exhibits characteristic diffraction signals at 26.25\({}^{\circ}\), 30.36\({}^{\circ}\) and 43.64\({}^{\circ}\) corresponding to (111), (200) and (220) planes of fcc crystal lattice structure. Similar fcc Pt/Pd peaks were observed for Pd/C, Ni@Pd, and Ni@Pd-Pt NCs. Herein, it is worth noting that the closely packed Pt/Pd (111) planes of experimental NCs did not change significantly. However, distinct variations in open planes (i.e., Pt/Pd (200) and (220)) have been observed for Ni@Pd and Ni@Pd-Pt NCs. As shown in Figure 2, (200) and (220) planes of Pd/C are negatively shifted (i.e., towards a lower angle), which indicates its larger lattice constant compared to Pt/C in open planes. This phenomenon is due to the oxide formation in open planes due to NaBH\({}_{4}\) assisted rapid crystal growth. For Ni@Pd NC, the closely packed (111) plane demonstrates similar features as that of Pd/C, however, compared to Pd/C, the (200) and (220) planes of Ni@Pd NC are significantly broadened and shifted to lower (larger lattice constant) and higher (lower lattice constant) angles, respectively. These characteristics indicate the co-existence of an expansive and compressive strain in the different planes. Moreover, Ni@Pd NC possesses higher background intensity (H) compared to Pd/C. The high background scattering is due to the presence of surface defects on Ni@Pd surface, which is consistent with former STEM analysis. Besides, the XRD pattern of Ni@Pd-Pt NC uncovers the effects of Pt-decoration on Ni@Pd surface. The Ni@Pd-Pt NC explores similar features in the closely packed (111) plane as that of Ni@Pd. However, compared to Ni@Pd, the broadened and narrowed (200) and (220) plane, respectively, indicates that Pt-clusters are mainly deposited on the open planes of Ni@Pd NC. The X-ray photoelectron spectroscopy (XPS) is utilized at Pd-3d orbitals to get further evidence about change in the electronic structure of Ni@Pd NC after Pt-clusters decoration. For acquiring the XPS spectra, the X-ray with a \(-\)2.6 nm probing depth and 485 eV excitation energy was used. The fitted XPS spectra of experimental NCs in the Pd-3d region has been represented in Figure 3. In a Pd\({}_{3\text{d}}\) spectrum, the doublet peaks centered around 336.0 eV and 341.0 eV, respectively, are characteristic current (photoemission) responses from Pd 3d\({}_{5\text{/2}}\) and Pd 3d\({}_{3\text{/2}}\) orbitals. These peaks were simulated  
Figure 2: X-ray diffraction (XRD) patterns of Ni@Pd-Pt NC compared with Ni@Pd, Pd/C and Pt/C.