The role of defects at low concentrations in the NH3/TiO2(110) adsorption system:: An Auger-photoelectron coincidence spectroscopy study

We present Auger-photoelectron coincidence spectra from the stoichiometric and reduced (by vacuum annealing) TiO2(110) surfaces after saturation exposure to NH3 at room temperature. The Ti 3p coincidence photoemission spectrum shows substantial emission from residual (or intrinsic) defects on the stoichiometric surface and additional emission from thermally-induced bridging oxygen vacancies on the reduced surface. Changes in the line shape of the coincidence Ti 3p spectrum upon adsorption show that NH3 binds to the five-fold coordinated Ti ions and the rosette structures of both surfaces, and at bridging oxygen vacancies of the reduced surface, but not at intrinsic defects. Quantitative analysis of the spectra supports a structural model where on the stoichiometric surface NH3 adsorbs at every other five-fold coordinated Ti site and at every other Ti site of the rosettes. On the reduced surface, NH3 adsorbs at bridging oxygen vacancies preventing adsorption on neighboring five-fold coordinated Ti sites, suggesting that the saturation coverage of NH3 is smaller for the reduced surface. We observe a correlation between the density of bridging oxygen vacancies and the coincidence intensity from five-fold coordinated Ti ions that gives an internally consistent account of these data. (C) 2000 American Institute of Physics. [S0021-9606(00)70447-8].