Water Oxidation through Interfacial Electron Transfer by Visible Light Using Cobalt-Modified Rutile Titania Thin-Film Photoanode.

TiO2 is a good photoanode material for water oxidation to form O2; however, UV light (λ < 400 nm) is necessary for this system to operate. In this work, cobalt species were introduced onto a rutile TiO2 thin film grown on a fluorine-doped tin oxide (FTO) substrate for visible-light activation of TiO2 and to construct water oxidation sites. TiO2 thin films were prepared on the FTO surface by the thermohydrolysis of TiCl4, followed by annealing at 723 K in air; the loading of the cobalt species was achieved simply by immersing TiO2/FTO into an aqueous Co(NO3)2 solution at room temperature, followed by heating at 423 K in air. Physicochemical analyses revealed that the cobalt species deposited on the TiO2 film was α-Co3(OH)4(NO3)2 and that the cobalt-modified TiO2 thin-film electrode had a visible-light absorption band that extended to 700 nm due to interfacial electron transitions from the cobalt species to the conduction band of TiO2. Upon anodic polarization in the presence of visible light, the cobalt-modified TiO2 thin-film electrode generated an anodic photocurrent with an onset potential of +0.1 V vs RHE, which was consistent with that of pristine rutile TiO2. Product analysis during the controlled potential photoelectrolysis in the presence of an applied bias smaller than 1.23 V under visible light showed that water oxidation to O2 occurred on the cobalt-modified TiO2/FTO. This study demonstrates that a visible-light-driven photoelectrochemical cell for water oxidation can be constructed through the use of earth-abundant metals without the need for a complicated preparation procedure.