Photoelectrochemical Water Oxidation Characteristics of Anodically Fabricated TiO2 Nanotube Arrays: Structural and Optical Properties

There are currently immense needs to optimize low-cost materials, such as TiO 2, so they can efficiently split water photoelectrochemically into hydrogen and oxygen, thus providing a clean energy fuel. To this end, the nature of the crystalline phase and the dimension of the photocatalyst are of crucial significance. In this study, films of 7 μm long titania nanotube arrays were fabricated via anodization of titanium foil in formamide electrolytes containing NH 4 F and H 3 PO 4 . Upon annealing the as-anodized nanotubes, the anatase-to-rutile phase transformation was found to start at 550 °C, which is about 120 °C above the temperature observed for the 500 nm long nanotube films, with the nanotube films remaining stable up to 580 °C. Analysis of the variation of crystallite size with annealing temperature along with XPS analysis of the films was used to investigate the reason behind this observation. UV−vis measurements showed that the absorption edges of the annealed samples were red shifted from that of the as-anodized sample. The stabilization of the anatase phase up to 550 °C, while keeping the tubular structure in place, is very significant as anatase is the most photoactive polymorph of titania. Besides, the 7 μm long nanotubular structure provides a large surface medium for light utilization through scattering. Used as photoanodes to photoelectrochemically split water, the 580 °C crystallized nanotube arrays showed a three-electrode photoconversion efficiency of 10% under UV illumination (100 mW/cm 2, 320−400 nm, 1 M KOH).