3.3 XPS analysis
The X-ray photoelectron spectroscopy (XPS) was used to investigate the surface chemical states of MCNS-10 composite. The Fig. 3aclearly reveals the existence of Mn, Cd, S, Ni and Se elements in MCNS-10 composite. Fig. 3b was the spectrum of Cd 3d, the binding energies located at 404.6 and 411.3 eV were respectively corresponded to Cd 3d5/2 and Cd 3d3/2, which were the Cd2+ ions [35]. The two binding energy of Mn 2p at 641 and 651.9 eV belong to the Mn 2p3/2 and Mn 2p1/2 respectively as shown in Fig. 3c . According to the S 2p spectra shown in Fig. 3d , the peaks at 160.9 (S 2p3/2) and 162.1 (S 2p1/2) eV were the characteristic peaks of S2- ions [33]. The deconvolution of the overlapped Ni 2p peaks four peaks (Fig. 3e ), the peaks located at 861.9 and 879.5 eV were respectively related to the satellite peaks of the Ni 2p1/2 and Ni 2p3/2 [37], and the peaks centered at 855.4 and 873.2 eV could be assigned to Ni2+ state [38]. The XPS spectrum of Se 3d represented in Fig. 3f could divided into three peaks, the main peaks located at 54.1 and 58.7 eV were respectively related to Se2- anions and high-valent oxide [39], and peak at 55 eV were ascribed to Se 3d5/2 of MCNS-10 [40]. These surface bonding states further proved that composite was composed of MCS and NiSe2.