Pretreatment dependence of adsorption properties of merocyanine dye at rutile (110) and (100) TiO ₂ surfaces studied by C K-Edge NEXAFS

The adsorption properties of n-TiO ₂ rutile (100) and (110) surfaces which were prepared by simply annealing, photoetching, and HF-annealing methods were investigated by using the C K-edge NEXAFS. It was revealed that the interaction between the substrate and the dye molecules on the simply annealed surface was stronger than that on the HF-annealed surface, whereas the net quantum efficiency of the dye-sensitized photocurrent on HF-annealed surface was larger than that of the simply annealed surface for both surface faces. These results indicated that the energy difference between the LUMO of Mc[18,1] and the conduction band of TiO ₂ was large enough to give a nearly 100% quantum efficiency of electron transfer from photoexcited dye to TiO ₂ even in the case of the simply annealed surface. The enhancement of photocurrent of the HF-annealed surface comparing with that of the simply annealed surface was explained by the assumption that HF-annealing process can remove inactive layers (or surface defects) on TiO ₂ substrate, which were not removed by the simply annealing process. On the other hand, the substrate-dye interaction of photoetched (110) surface was close to that on atomically flat (100) surface and largely different from that on the atomically flat (110) surface, whereas there is no difference in the substrate-dye interaction between photoetched and atomically flat (100) surfaces. This was mainly due to the generation of the (100) face on the surface by the photoetching process, regardless of the crystal faces of the substrates. It was revealed that the inactive layer (or surface defects) was also removed during the photoetching procedure. The IPCE value for the photoetched surface was much larger than that of HF-annealed surface, which may be attributed to not only larger amount of dye molecules (due to the larger surface area) but also rearrangement of dye molecules due to the characteristic morphology of photoetched surfaces.