The surface structure of titania and the effect of reduction

Recent developments in the elucidation of the structure of the titania surface are reviewed, and they reveal a rich panoply of behaviour. The pristine TiO2(1 1 0) surface shows the expected (1 × 1) bulk termination, but very mild deviations from stoichiometry of the bulk have significant effects on the surface structure which evolves successively with increasing reduction to (1 × 3), (1 × 2), ‘cross-linked’ (1 × 2), the appearance of crystallographic shear planes, and eventually to gross surface roughening due to a high density of the latter features. This reduction is induced by thermal treatment in vacuum or by sputtering. STM has shown that reduced surface species, such as Ti3+, which are present at low temperatures after sputtering, are lost by dissolution into the bulk of the sample, and this process begins to occur at ∼700 K. In oxidation the reverse occurs above about 600 K, that is, titanium cations return to the surface where they are re-oxidised and grow new surfaces of titania in a layer-by-layer fashion. STM has revealed that anion vacancies can remain in the surface region if the sample is not annealed to high temperatures. These have recently been shown to consist of missing oxygen atoms in the [0 0 1]-strings of surface oxygen and are very reactive to gas phase oxidants, including oxygen and water.