Simply tuned and sustainable cobalt oxide decorated titania nanotubes for photoelectrochemical water splitting

Effective utilization of solar energy is an urgent global issue for solving current environmental problems and the energy crisis. A challenging issue in water splitting for solar fuel production is developing high performance and scalable photo/electro catalysts using low cost approach. Here, CoO _x nanostructures were grown on anodized TiO ₂ nanotube arrays (TNAs) by amperostatic electrodeposition method as oxygen evolution reaction (OER) photoanodes. The effects of electrodeposition current density and deposition time on the growth of so-called nanostructures were investigated based on photoelectrochemical (PEC) measurements and structural characterizations. X-ray diffraction patterns of the photoanodes revealed formation of stoichiometric anatase TiO ₂ as well as cobalt oxide on the surface. Based on scanning electron microscope, by increasing the electrodeposition current, the morphology of CoO _x nanostructures changed which makes electrodeposition current of 0.1 mA·cm ⁻² resulting in optimum distribution of cobalt oxide electrocatalysts with average size of 35 nm which exhibit the highest photocurrent density due to efficient and uniform distribution of electrocatalysts with no screening effect. Electrodeposition time was found to have a linear relationship with the surface coverage of TNAs by the electrocatalyst, and 2000s, having long charge carrier lifetime and successfully adjusted electrocatalyst to photocatalyst amount ratio turned out to be the optimum electrodeposition time. In this work, we achieved optimized electrodeposition conditions (current density and deposition time) which improve the PEC performance of CoO _x -TNAs by 2 times higher than the pristine TNA photocatalyst. Photovoltage measurement and conductive atomic microscopy along with absorption spectra of the samples revealed formation of visible active p-type CoO _x species in interface with n-type titania nanotube.