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