Co Substitution Effect on the Electronic Structure of NiS Studied by X-Ray Photoemission Spectroscopy

X-ray photoemission spectroscopy (XPS) measurements of the valence band, Ni 2p core level, Co 2p core level and S 25, 2p core levels were carried out on the metallic state of Ni-ν.Coν.S (0 ≤ ν ≤ 0.06). The main peak of the valence band spectra shifts toward lower binding energy (EB) with increasing ν (dEB/dν = -2.6-0.4 eV/mol). This behavior is different from those reported on vacancy introduction or on the Se substitution in Ni1_xS or NiS1_YSeY (dEB/dx ≈0 dEB/dy≈0). The Ni 2p3/2 core level peak also shifts toward lower EB (dEB/dν= -1.8-0.6 eV/mol), while no change in the S 2s or 2p core level peak is observed. We propose the possibility that the hybridization between the Co d7 L and the Ni d8 L peaks near the EF, where L denotes a ligand hole, leads to the modification of the electronic state of Ni sites in Ni1-ν CoνS.