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

Abstract

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.