Photoemission study of the modification of the electronic structure of transition-metal overlayers on TiO2 surfaces III. Ni on TiO2(0 0 1) and Cu on TiO2(1 1 0)

The electronic states of the Ni overlayers on TiO2(0 0 1) surface and Cu overlayers on TiO2(1 1 0) surface have been investigated using angle-resolved and resonant photoemission spectroscopy with synchrotron radiation. In the Ni/TiO2(0 0 1) system, an in-gap emission from the reduced Ti3+ 3d1 state appears at 1.2 eV below EF for the initial Ni deposition. Upon further deposition, the emission grows and is evolved to a spectrum of Ni metal, with forming no Fermi edge, suggesting the formation of small Ni clusters on TiO2(0 0 1). In the Cu/TiO2(1 1 0) system, Cu grows in a Stranski–Krastanov mode. Two in-gap emissions from the reduced Ti3+ 3d1 state and oxidized Cu+ 3d10 or Cu2+ 3d9 state are observed at 0.8 and 2.8 eV below EF, respectively, for the initial Cu deposition, giving convincing evidence that there is significant charge transfer between Cu and the substrate. Upon further deposition, the emission in the band-gap region is evolved to a spectrum of Cu metal, though delayed completion of the Fermi edge is observed. Such delay can be explained by finite cluster size effects.