A study on the spectral photoresponse and photoelectrochemical properties of flame-annealed titania nanotube-arrays

We have examined the spectral photoresponse and photoelectrochemical properties of novel highly-ordered TiO2 nanotube-array photoelectrodes, made by anodization of a titanium foil, as a function of nanotube-array aspect-ratio and propane flame annealing. One geometry consisted of high aspect ratio (similar to 50) nanotube-arrays 4.4 mu m in length, while the other geometry comprised shorter nanotube-arrays, approximately 0.2 mu m in length, having a smaller aspect ratio (similar to 10). The propane flame annealing significantly enhances the visible spectrum absorption of the short nanotubes (NTs), as well as their solar-spectrum induced photocurrents, while not significantly affecting the absorption spectrum of the longer NTs. X-ray photoelectron spectroscopy shows that flame annealing increases the carbon content in the NTs of both geometries. For visible spectrum illumination incident photon to charge carrier efficiencies up to 5% were recorded for the flame annealed samples, while a maximum photocurrent density of 1.5 mA cm(-2) was obtained under simulated solar spectrum AM 1.5 illumination at high anodic polarization in 1 M KOH. Comparative results of the I-V characteristics, photon conversion efficiencies, composition and the surface morphology of these arrays are discussed.