Role of surface stoichiometry on the interfacial electron behavior at Ni/TiO_2(0 01) interfaces

摘要

The interfacial properties of Ni clusters grown on the stoichiometric and reduced rutile TiO_2(001) surfaces were investigated by means of X-ray photoelectron spectroscopy (XPS). The binding energies (BE's) of elements from both overlayers and substrates were found to be affected by the formation of interfacial dipole. Regardless of the TiO_2 surface stoichiometry, the Ni 2p_(3/2) BE's move monotonically toward lower value with the increase of Ni thickness due to the cluster size effect. However, the Ni 2p_(3/2) BE shift is much smaller on reduced TiO_2(0 01) surfaces compared to that on the annealed stoichiometric surface. For stoichiometric and lightly reduced TiO_2 surfaces, O 1s BE's exhibit an unexpected upward shift with increasing Ni thickness below 2 A, and then downward shift to lower BE's when the Ni thickness increases further. This opposite tendency is attributed to the insulator-to-metal transition. On heavily reduced surface, only monotonically downward shiftofthe O 1s BE's 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.