Structure sensitivity and photocatalytic reactions of semiconductors. Effect of the last layer atomic arrangement.

Photocatalytic reactions of heterogeneous systems, probably the most complex catalytic reactions, are central to a wide variety of phenomena including those of evolution theory, energy conversion, and environmental cleanup. Although catalytic reactions are essentially surface driven, the effect of bulk structure is particularly important for the photo processes in solid materials because it directly affects the excitation and charge transfer mechanisms. To treat surface photocatalytic reactions, it is necessary to differentiate between surface and bulk effects. We have successfully decoupled the surface-bulk effects by using a TiO2 (001) single crystal. This was achieved by allowing for surface reconstruction and measuring the rate of reaction for the two thermodynamically stable reconstructed surfaces of the oxide single crystal in ultrahigh vacuum conditions. A considerable difference, in the quantum yield of the photoreaction of acetic acid, between the two reconstructed surfaces was found. This large difference (due to different surface electron-hole recombination rates) between both reconstructed surfaces could not be seen without decoupling the surface from bulk effects.