Role of surface stoichiometry on the interfacial electron behavior at Ni/TiO2(0 0 1) interfaces

The interfacial properties of Ni clusters grown on the stoichiometric and reduced rutile TiO2(0 0 1) surfaces were investigated by means of X-ray photoelectron spectroscopy (XPS). The binding energies (BEʹs) of elements from both overlayers and substrates were found to be affected by the formation of interfacial dipole. Regardless of the TiO2 surface stoichiometry, the Ni 2p3/2 BEʹs move monotonically toward lower value with the increase of Ni thickness due to the cluster size effect. However, the Ni 2p3/2 BE shift is much smaller on reduced TiO2(0 0 1) surfaces compared to that on the annealed stoichiometric surface. For stoichiometric and lightly reduced TiO2 surfaces, O 1s BEʹs exhibit an unexpected upward shift with increasing Ni thickness below 2 Å, and then downward shift to lower BEʹs when the Ni thickness increases further. This opposite tendency is attributed to the insulator-to-metal transition. On heavily reduced surface, only monotonically downward shift of the O 1s BEʹs was observed with the increase of Ni thickness. The different behaviors are well elucidated by collective contributions of interfacial charge transfer and image charge effect.