Chemical Bonds and Charge-Transfer Dynamics of a Dye-Hierarchical-TiO2 Hybrid Interface

The adsorption of Zn-tetraphenylporphyrin (ZnTPP) on nanoporous hierarchically organized anatase TiO2 structures and the properties of the corresponding hybrid interface were studied by synchrotron radiation experiments. The molecular structure, electronic properties, and bonding with nanostructured TiO2 surfaces were analyzed by photoemission (XPS and UPS) and X-ray absorption spectroscopy (XAS). The charge transfer at the interface was investigated by means of valence band resonant photoemission experiments (ResPES) at the C K-edge. We show that the charge-transfer dynamics between the photoexcited ZnTPP and TiO2 is strongly influenced by the presence of defects on the TiO2 surface. On a stoichiometric anatase nanostructure, ZnTPP bonding occurs primarily via carbon atoms belonging to the molecular phenyl rings, and this creates a preferential channel for the charge transfer. This phenomenon is reduced in the case of defective TiO2 surface, where ZnTPP interacts mainly through the molecule macrocycle. Our results represent a surface science study of the dye molecule behavior on a nanoporous TiO2 photoanode relevant to dye-sensitized or hybrid solar cell applications, and they show the importance of the surface oxidation state for the charge-transfer process.