Electronic properties and interface characterization of phthalocyanine and Ru-polypyridine dyes on TiO2 surface

Electronic structures, energy levels alignment and electronic interactions of a series of phthalocyanine and Ru-polypyridine dyes at the interfaces of dyes/TiO2 films have been studied by photoelectron spectroscopy. The shifts of carboxylic C 1s core level binding energies of carboxylated dye complexes at different adsorption states indicate that the dyes are mainly adsorbed on the nanoporous TiO2 surface via carboxylic groups. The relative positions of the highest occupied molecular orbitals/the lowest unoccupied molecular orbitals (HOMO/LUMO) of these phthalocyanine and Ru-polypyridine dyes with respective to the conduction band (CB) of TiO2 were determined from a combination of X-ray and ultraviolet photoemission (XPS and UPS) measurements and further compared with their corresponding electrochemical oxidation potentials. Both measurements support the general picture of Ru-polypyridine complexes that their HOMO has predominately core metal Ru 4d(t2g) character while for phthalocyanine dyes the HOMO energy levels are more related to their chromophores.