Oxygen-induced restructuring of the TiO2(110) surface: a comprehensive study

We report a comprehensive experimental and theoretical study of the effect of oxidizing a TiO2(110) surface at moderate temperatures. The surfaces are investigated with scanning tunneling microscopy (STM), low-energy He+ ion scattering (LEIS) and static secondary ion mass spectroscopy (SSIMS). Flat( 1 x 1)-terminated TiO2(110) surfaces are obtained by sputtering and anneali ng in UHV at 880 K. These surfaces are exposed to oxygen gas at elevated te mperatures in the range 470-830 K. Formation of irregular networks of pseud o-hexagonal rosettes (6.5 Angstrom x 6 Angstrom) and small (1 (1) over bar 0] oriented (1 x 1) islands along with {001}-oriented strands is induced at temperatures from 470 to 660 K. After annealing above 830 K, only regular (1 x 1) terraces and white strands are observed. The composition of these o xygen-induced phases is quantified using O-18(2) gas in combination with LE IS and SSIMS measurements. The dependence of the restructuring process on a nnealing time, annealing temperature, and sample history is systematically investigated. Exposure to (H2O)-O-18 and air in the same temperature regime fails to induce the restructuring. UHV annealing of restructured, oxygen-e nriched TiO2(110) surface smooths the surfaces and converts the rosette net works into strands and finally into the regular (1 x 1) terraces. This is r eported in an accompanying paper [M. Li, W. Hebenstreit, U. Diebold, Phys. Rev. B (1999), submitted]. The rosette model is supported by first-principl es density functional calculations which show a stable structure results, a ccompanied by significant relaxations from bulk-truncated positions. A mech anism for the dynamic processes of the formation of rosettes and (1 x 1) is lands is presented and the importance of these results for the surface chem istry of TiO2(110) surfaces is discussed. (C) 1999 Elsevier Science B.V. Al l rights reserved.