The TiO₂-catechol complex: Coupling type II sensitization with efficient catalysis of water oxidation

Two main requirements must be fulfilled in order to construct an efficient TiO₂-based photo-electrochemical water splitting cell. One is the expansion of the cell's spectral response, usually by the attachment of a sensitizing dye monolayer on the surface of the TiO₂. The second involves the incorporation of a water oxidation catalyst that reduces the overpotential for the oxygen evolution reaction. These requirements are often achieved by the co-adsorption of both the dye and the catalyst on the TiO ₂, or by a covalent attachment of the catalyst to the dye molecule. Here, the possibility to use a single material that acts as a sensitizer and a catalyst is presented. The use of a catechol molecule to form a type II charge transfer complex with TiO₂ widens the absorption of the system into the visible region. The TiO₂-catechol complex is highly catalytic toward the oxidation of water to oxygen, reducing the electrocatalytic reaction overpotential by 500 mV compared to bare TiO₂. A suggested catalytic mechanism for the water oxidation reaction is described. This methodology opens a new path for type II charge transfer complexes to be utilized as catalysts/light absorbers in water splitting systems based on TiO₂ or other metal oxides. The role of TiO₂-catechol type II charge transfer complex in the photoelectrochemical water oxidation is investigated. The formation of the TiO₂-catechol complex expands the spectral response of the system far into the visible region. Additionally, the complex serves as a hole reservoir that catalyzes the electrochemical oxygen evolution reaction.