Unconventional synthesis of titanium-tetrapyrrole films at TiO2(110): a computational and experimental approach

Surface modification with functional films is a popular way to improve the properties of inorganic particles for a number of applications. A well-known case is that of dye-sensitized cells, where absorption of the polar spectrum in the visible and near-infrared region of metal oxides such as TiO2 or ZnO is enhanced by films of metal complexes with suitable HOMO-LUMO gaps. It is generally assumed that in these hererostructures the supporting metal oxide is chemically innocent, and that molecules are adsorbed - chemically or physically - unchanged. In contrast to this picture, we show that, starting from room temperature, TiO2 actively interacts with adsorbed porphyrins, exchanging atoms, and giving rise to self-hydrogenations, selfmetalations, and dehydrogenations reactions, which ultimately bring to the synthesis of films of exceptionally high thermal stability (up to to 450 degrees C), potentially useful for sensors and photocatalysis applications in harsh environments. Furthermore, our findings indicate that metal exchange with the very reactive Ti atoms must be taken into account when designing porphyrinsensitized solar cells, because the critical temperature for the onset of self-metalation is very close to the normal operating temperature of photovoltaic devices.