Oxygen-induced restructuring of rutile TiO2(110): formation mechanism, atomic models, and influence on surface chemistry

The rutile TiO2(110) (1x1) surface is considered the prototypical 'well-def ined' system in the surface science of metal oxides. Its popularity results partly from two experimental advantages: (i) bulk-reduced single crystals do not exhibit charging, and (ii) stoichiometric surfaces, as judged by ele ctron spectroscopies, can be prepared reproducibly by sputtering and anneal ing in oxygen. We present results that show that this commonly applied prep aration procedure may result in a surface structure that is by far more com plex than generally anticipated. Flat, (1x1)-terminated surfaces are obtain ed by sputtering and annealing in ultrahigh vacuum. When re-annealed in oxy gen at moderate temperatures (470-660 K), irregular networks of partially c onnected, pseudohexagonal rosettes (6.5x6 Angstrom wide), one-unit cell wid e strands, and small (approximate to tens of Angstrom) (1x1) islands appear . This new surface phase is formed through reaction of oxygen gas with inte rstitial Ti from the reduced bulk. Because it consists of an incomplete, ki netically limited (1x1) layer, this phenomenon has been termed 'restructuri ng'. We report a combined experimental and theoretical study that systemati cally explores this restructuring process. The influence of several paramet ers (annealing time, temperature, pressure, sample history, gas) on the sur face morphology is investigated using STM. The surface coverage of the adde d phase as well as the kinetics of the restructuring process are quantified by LEIS and SSIMS measurements in combination with annealing in O-18-enric hed gas. Atomic models of the essential structural elements are presented a nd are shown to be stable with first-principles density functional calculat ions. The effect of oxygen-induced restructuring on surface chemistry and i ts importance for TiO2 and other bulk-reduced oxide materials is briefly di scussed.