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 2 p core level, Co 2 p core level and S 2 s , 2 p core levels were carried out on the metallic state of Ni 1- v Co v S (0≤ v ≤0.06). The main peak of the valence band spectra shifts toward lower binding energy ( E B ) with increasing v ( d E B / d v =-2.6±0.4 eV/mol). This behavior is different from those reported on vacancy introduction or on the Se substitution in Ni 1- x S or NiS 1- y Se y ( d E B / d x ≈0; d E B / d y ≈0). The Ni 2 p 3/2 core level peak also shifts toward lower E B ( d E B / d v =-1.8±0.6 eV/mol), while no change in the S 2 s or 2 p core level peak is observed. We propose the possibility that the hybridization between the Co d 7 L and the Ni d 8 L peaks near the E F , where L denotes a ligand hole, leads to the modification of the electronic state of Ni sites in Ni 1- v Co v S.