First Principles Study on the Interfacial Structure and Electronic Properties of a Metal-Free Organic Dye/TiO₂ Photoanode for Water Oxidation

Recently, metal-free organic dyes have been successfully incorporated in dye-sensitized water oxidation photoanode utilizing red light. In order to understand the interfacial structures and the related electronic/optical properties of the dye-sensitized water oxidation photoanode at nanoscale level, a metal free porphyrin dye/TiO₂ interface that has been applied in a water splitting system experimentally is modelled via first principles. The DFT calculations predict vertical adsorption of the organic dye onto the TiO₂ substrate via its carboxylic acid anchor. The calculated UV–Vis absorption spectra of the porphyrin organic dye/TiO₂ system demonstrate peak absorption wavelengths at ca. 500, 580, and 620 nm, consistent with the experiments. Detailed electronic structure analysis including band structure, density of states and orbital distributions suggests effective dye sensitization and interfacial electron transfer in the organic dye/TiO₂ system in the water oxidation photoanode, which is essential for water splitting efficiency.