Elucidating the Many-Body Effect and Anomalous Pt and Ni Core Level Shifts in X-ray Photoelectron Spectroscopy of Pt-Ni Alloys

The local structure, many-body effect, and charge redistribution of Pt and Ni in Pt-Ni alloys (Pt-Ni = 3:1, 1:1, and 1:3) have been studied by X-ray absorption spectroscopy and X-ray photoelectron spectroscopy (XPS). It is found that upon alloying, the skew many-body line shape of Pt 4f peaks in Pt metal becomes more symmetric in Pt alloys, while the associated Ni 2p peaks become more asymmetric and are accompanied by more intense shake-up satellites. This is because Pt gains while Ni loses valence d electrons, resulting in different phase shift of electrons scattered near the Fermi level. Meanwhile, the redistribution of the density of states (DOS) is confirmed by experiment and density functional theory calculations, suggesting that the centroid Ni d states tend to shift closer to the Fermi level, while Pt d states are down-shifted in alloys. This DOS redistribution causes the anomalous binding energy shift for both Pt and Ni core levels in XPS, which has not been well explained in the literature. These findings also suggest that the d-band centroid shift originates from two different mechanisms for Pt-based alloys and Pt-skinned core-shell catalysts.