Enhanced visible light photocurrent generation at surface-modified TiO2 nanotubes

Photoelectrodes consisting of TiO2 nanotube layers with different thicknesses (0.5 μm, 1.7 μm, 3 μm, 6 μm, 9 μm, and 18 μm) were prepared by anodization of titanium substrates and subsequent surface modification by a heat treatment at 400 °C in the presence of urea pyrolysis products. In contrast to unmodified TiO2 nanotubes, the modified photoelectrodes exhibit photocurrents under visible light irradiation down to 750 nm. Photocurrent transients indicate enhanced recombination unless a suitable hole-scavenger, like iodide, is present since the photogenerated holes do not oxidize water efficiently. In the visible light the photoconversion efficiency increases significantly with nanotube length. The maximum incident photon-to-current efficiency (IPCE) was observed for tubes with the length of 6–9 μm (IPCE ∼4.5% and 1.4% at 450 nm and 550 nm, respectively) and the photocurrent enhancement with increasing tube length is found to be stronger at longer irradiations wavelengths.