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 TiO2, 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 mu m long titania nanotube arrays were fabricated via anodization of titanium foil in formamide electrolytes containing NH4F and H3PO4. Upon annealing the as-anodized nanotubes, the anatase-to-rutile phase transformation was found to start at 550 degrees C, which is about 120 degrees C above the temperature observed for the 500 nm long nanotube films, with the nanotube films remaining stable up to 580 degrees 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 degrees C, while keeping the tubular structure in place, is very significant as anatase is the most photoactive polymorph of titania. Besides, the 7 mu m long nanotubular structure provides a large surface medium for light utilization through scattering. Used as photoanodes to photoelectrochemically split water, the 580 degrees 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).