MIES and UPS(HeI) studies on reduced TiO₂(110)

Abstract

We have studied reduced TiO 2 (110) surfaces by combining metastable impact electron spectroscopy (MIES) and UPS(HeI). The reduced Ti species were preparation-induced: their number density was modified either by adsorption of K atoms or by a combined annealing/oxygen exposure procedure. The emission from the bandgap state (binding energy 0.9 eV), caused by reduced Ti 3+ 3d species, was monitored. Bandgap emission is seen clearly with UPS(HeI) and thus can be used to monitor the number density of the nearsurface reduced species. A corresponding spectral structure cannot be seen with MIES. We propose that the excess charge density introduced either by preparation-induced oxygen vacancies or by K adsorption is delocalized over several surface and subsurface Ti sites; this, together with the partial shielding of the reduced Ti species, prevents detection of the reduced Ti species with MIES.The re-oxidation and restructuring of the reduced TiO 2 (110) surface, caused by simultaneous oxygen exposure and annealing, was studied at temperatures between 400 and 770 K, again by recording the Ti 3+ 3d emission (0.9 eV bandgap state) with UPS(HeI). The surface can be completely re-oxidized by oxygen exposure at any selected annealing temperature in the range given above. Morphology changes, leading to a partially reduced surface, take place when the re-oxidized surface is further annealed at T > 600 K under reducing conditions. The results give support to the assumption that the re-oxidation is caused by the growth of additional titania whereby the Ti stems from the bulk and the oxygen originates from the gas. The restructuring of the re-oxidized surface upon annealing under reducing conditions appears to be due to diffusion of Ti interstitials to the surface.

Interaction of Au with titania: the role of reduced Ti

“…These Ti 3+ surface defects exhibit a valence band feature with a binding energy of 0.9 eV, as evidenced in figure 1(C). This defect can be completely re-oxidized by exposure to oxygen, as demonstrated by UPS where the Ti 3+ state at 0.9 eV disappears completely (see figure 1(D)) [34]. Following the deposition of Au onto the reduced titania surface, a significant decrease in the intensity of the Ti 3+ state is apparent, showing that Au initially nucleates at the defect sites.…”

Catalytically Active Gold: From Nanoparticles to Ultrathin Films

“…These Ti 3+ surface defects can be completely reoxidized by exposure to oxygen. 35 Following Au deposition onto the reduced titania surface, a decrease in the intensity of the Ti 3+ state is apparent, showing that Au initially nucleates at the defect sites. This suggests that Au atoms bond at oxygen vacancy sites, that is, bonding between Au and Ti 3+ .…”

Electronic structure of point defects in controlled self-doping of theTiO2(110) surface: Combined photoemission spectroscopy and density functional theory study

“…24 These studies show that reduced species appear in XPS as a clear shoulder on the notionally Ti 4+ bulk signal ͑chemical core level shift of Ti 2p states͒ 25 and in UPS as the appearance of a Ti 3d derived defect state in the band gap. 26 This state is found in the band gap, 0.9 eV below the Fermi level, 12,[14][15][16][17][18]26 and is attributed to formation of reduced Ti 3+ . The Fermi level in nonstoichiometric TiO 2 is pinned to the bottom of the conduction band ͑CB͒, which means that the separation from the defect state to the Fermi level is essen-tially the same as the separation to the CB.…”