Origin of XPS binding energy shifts in Ni clusters and atoms on rutile TiO2 surfaces

Cluster-size-dependent binding energy (BE) shifts of Ni 2p(3/2) spectra in Ni clusters with respect to bulk Ni metal have been studied as a function of Ni coverage on clean rutile TiO2(001) and TiO2(110) surfaces at room temperature. As a common method to distinguish initial and final state contributions to the core-level binding energy shifts in clusters, Auger parameter (AP) analysis of photoelectron spectra has been employed and reveals an obvious initial state contribution at the coverage of 0.5 monolayers (ML). From a comparison of results for TiO2(001) and (110) surfaces, the initial state effect is demonstrated to be strongly affected by the substrate and is assigned to a combination of eigenvalue shift in surface core-level shift (SCLS) and charge transfer between the metal clusters and substrates. The Ni 2p(3/2) BE's of atomic Ni on TiO2(001) and (110) surfaces are deduced to be 853.69 and 853.55 eV, respectively, from an extrapolation of the experimental BE curves to zero Ni coverage. Compared with atomic Ni in gas phase, relaxation shifts of 7.34 and 7.48 eV are obtained on TiO2(001) and (110) surfaces, respectively. These values are very close to the relaxation shift of 7.3 eV due to d electron screening, indicating d-like screening effects from the TiO2 substrates after Ni 2p photoionization. (C) 2008 Elsevier B.V. All rights reserved.