Dye-sensitized photocurrents and adsorption properties of merocyanine dye at atomically flat rutile (1 1 0) and (1 0 0) TiO2 surfaces

The crystal-face dependence of the dye-sensitized photocurrents and the adsorption properties of benzothiazole merocyanine (Mc[18,1]) dye molecules were investigated, using atomically flat (1 0 0) and (1 1 0) TiO2 single crystal surfaces. From the estimation of the amount of the transferred charge from the TiO2 surface to CO groups of dye molecules based on NEXAFS data, it was revealed that the interaction of the adsorbed molecules and the (1 1 0) surface was much stronger than that for the (1 0 0) surface. On the other hand, the absorbed photon to current conversion efficiency (APCE) value was almost the same for both surfaces. We suggested a possible explanation as follows: the energy difference between the LUMO of Mc[18,1] and the conduction band of TiO2 was large enough to give a nearly 100% quantum efficiency of electron transfer from photoexcited dye to TiO2, which made the difference in the interaction between dye molecules and TiO2 not apparent. The incident photon to current conversion efficiency (IPCE) for the (1 0 0) surface was much larger than that for the (1 1 0) surface, which was explained by the fact that the amount of the adsorbed dye molecules on the (1 0 0) surface was larger than on the (1 1 0) surface, probably due to the larger surface density of five-coordinated Ti sites in the former surface.