One step towards bridging the materials gap: Surface studies of TiO ₂ anatase

We present a short overview of surface studies on the main low-index surfaces of anatase, the technologically most interesting crystallographic form of titanium dioxide. Results are compared to the extensively investigated surfaces of TiO₂ rutile. The anatase (1 0 1) surface is stable in a (1 × 1) configuration. It exhibits twofold coordinated (bridging) oxygen atoms and fivefold coordinated Ti atoms with a density comparable to the one found on rutile (1 1 0). Step edges are terminated by fourfold coordinated Ti sites. In contrast to rutile (1 1 0), anatase (1 0 1) does not show a strong tendency for losing twofold coordinated oxygen atoms upon annealing in ultrahigh vacuum. The apparent lack of point defects is also reflected in the adsorption/desorption behavior of water and methanol. The anatase (1 0 0) surface has the second-lowest surface energy and tends to from a (1 × 2) reconstruction. A model with (101)-oriented microfacets agrees with the observed features in atomically-resolved STM images. The (0 0 1) surface forms a (1 × 4) reconstruction that is well explained by an 'ad-molecule' model predicted from density functional theory calculations. A (1 × 3) reconstruction was observed for the anatase (1 0 3) surface.