Titania–Silica Interfaces

Abstract

The photocatalytic activity of titanium dioxide is sometimes observed to increase upon addition of silica, but the origin of this enhancement is not clear. To shed light on this effect, we investigate the titania/silica system in case of perfect segregation using density functional theory. Two situations have been considered: a single silica monolayer covering a titania surface and a bulk titania/silica interface. In both cases, we find that the presence of silica strongly modifies the electronic structure of the catalyst. In the case of the bulk interface, the analysis of the projected density of states reveals that interface electronic states give a large contribution to the edges of valence and conduction bands. For a silica monolayer on a (101) surface of anatase, the spatial localization of the states near the edge of the valence band depends on the hydroxylation state of the monolayer. Similar to the bulk case, the hydrogen-free monolayer hosts interface states, whereas in the fully hydroxylated system, these disappear and all top valence band states are located inside the titania slab. These results can be rationalized in terms of the number of Ti atoms available for bonding with bridging oxygens at the interface, suggesting that the ratio of Ti–O–Si and 2Ti–O–Si linkages allowed by the interface bonding topology may play an important role in photoabsorption processes.