Rutile Titania Particulate Photoelectrodes Fabricated by Two-Step Annealing of Titania Nanotube Arrays

TiO2 nanotube arrays fabricated by anodization and post annealing are extensively studied as an n-type semiconductor electrode for photoelectrochemical (PEC) water splitting owing to their large surface area and efficient electron transport property. The rutile phase is believed to be an inactive component of the TiO2 nanotube's electrode because annealing at high temperatures decreases the PEC efficiency with the transformation of anatase into rutile crystallites. In contrast, herein, we found that photoelectrodes prepared by two-step annealing, in which TiO2 nanotubes annealed in air at 650°C were then treated in a nitrogen atmosphere at a higher temperature, exhibited higher PEC efficiency despite the anatase nanotube structure changes to rutile particles. Sheet resistance measurement and Mott–Schottky analysis showed that the enhanced efficiency is attributed to a significant increase in donor density by partial reduction of rutile TiO2. The second annealing in the reductive atmosphere is essential to provide a columnar arrangement of TiO2 crystallites with high donor density resulting in high PEC properties for the oxidation of water to O2. This suggests that improving the electron transport property by the enhanced electrical conductivity and the interconnected nanocrystalline structure is important to enhance the PEC property of rutile TiO2 particulate photoanodes.