## 3 Results and Discussion  
### Synthesis and Structural Characterization  
Ni\({}_{3}\)Se\({}_{4}\) powders were synthesized by a facile and low-temperature method in ethanol (solvent), and the composites were obtained by growing MoSe\({}_{2}\) on the as-synthesized Ni\({}_{3}\)Se\({}_{4}\) nanosheets as depicted in Figure 1. The structural properties of Ni\({}_{3}\)Se\({}_{4}\) and Ni\({}_{3}\)Se\({}_{4}\)-MoSe\({}_{2}\) powders were investigated by XRD. As displayed in Figure 2, three dominant peaks appeared at 33.1\({}^{\circ}\), 44.8\({}^{\circ}\), and 50.6\({}^{\circ}\) in the spectra of Ni\({}_{3}\)Se\({}_{4}\), which were assigned to the (312), (514), and (310) crystal faces of the Ni\({}_{3}\)Se\({}_{4}\) phase, respectively (PDF card number 18-0890) [43]. Further, the obtained diffraction peaks of MoSe\({}_{2}\) can be indexed to the hexagonal 2H-MoSe\({}_{2}\) (JCPDS 29-0914). The XRD patterns of Ni\({}_{3}\)Se\({}_{4}\)-MoSe\({}_{2}\) were like that of Ni\({}_{3}\)Se\({}_{4}\), but (002) peak assigned to the basal plane of MoSe\({}_{2}\) could still be observed in the patterns. Further, XPS analysis was carried out to explore the composition and chemical state of the composites. The survey spectrum of the Ni\({}_{3}\)Se\({}_{4}\)-MoSe\({}_{2}\) composites clearly shows the peaks of Ni 2p, Mo 3d, Se 3d, and O 1s (Figure 3a). The O 1s peak occurred owing to the unavoidable oxidation that takes place during the synthesis process. To confirm the oxidation states of these three elements in the composites, the high-resolution spectra of Ni 2p, Mo 3d, and Se 3d were also obtained (Figure 3b). The spectrum of Ni 2p in the Ni\({}_{3}\)Se\({}_{4}\)-MoSe\({}_{2}\) composites could be deconvoluted into two doublets (2p\({}_{3/2}\) and 2p\({}_{1/2}\)) along with two shake-up satellites, which appeared due to the spin-orbit coupling effect [44]. Specifically, the 2p\({}_{3/2}\) peak could be further deconvoluted into two components, which were located at the band energies of 853.6 and 855.8 eV. The lower energy band could be attributed to the +2 valence state of nickel (Ni\({}^{2+}\)), whereas the higher energy could be attributed to the +3 valence state of nickel (Ni\({}^{3+}\)) [43; 45; 46]. Similarly, the 2p\({}_{1/2}\) peak could also be resolved into two components that were located at the band energies of 870.9 and 873.5 eV, which were assigned to Ni\({}^{2+}\) and Ni\({}^{3+}\), respectively [43; 45; 46]. Further, the satellite peaks appeared at a binding energy slightly positive to the peaks of Ni 2p\({}_{3/2}\) and Ni 2p\({}_{1/2}\)[45]. Compared to the spectrum of Ni 2p in pure Ni\({}_{3}\)Se\({}_{4}\), all components and satellite peaks shifted slightly (approximately 0.1-1.2 eV) towards lower binding energy, which indicated the chemical bonding between Ni\({}_{3}\)Se\({}_{4}\) and MoSe\({}_{2}\). For the spectra of Mo 3d (Figure 3c), the two peaks at binding energies of 229.0 and 232.2 eV were assigned to Mo 3d\({}_{5/2}\) and Mo 3d\({}_{3/2}\) of Mo (IV), confirming the presence of Mo\({}^{4+}\), while the peak at binding energies of 235.6 eV was probably due to the Mo oxide [47], which was formed by oxidation of the metal Mo during the synthesis process. In addition, the Se 3d spectra of the Ni\({}_{3}\)Se\({}_{4}\)-MoSe\({}_{2}\) composites is shown in Figure 3d. The peaks at binding energies of 54.3 and 55.2 eV were corresponded to Se 3d\({}_{5/2}\) and Se 3d\({}_{3/2}\), respectively, confirming the presence of Se in -2 valence state, whereas the peak at binding energies of 59.9 eV oxidized Se species (SeO\({}_{x}\)). Further, the peaks of both Se 3d\({}_{5/2}\) and 3d\({}_{3/2}\) in the composites shifted slightly towards a higher binding energy, whereas the position of the SeO\({}_{x}\) peak remained unchanged. Figure 3: XPS profiles (**a**) survey, (**b**) Ni 2p, (**c**) Mo 3d, and (**d**) Se 3d. The morphologies and structures of the Ni\({}_{3}\)Se\({}_{4}\) and Ni\({}_{3}\)Se\({}_{4}\)-MoSe\({}_{2}\) composites with different amounts of Ni\({}_{3}\)Se\({}_{4}\) were characterized by FE-SEM and TEM.