Black TiO2 Nanotubes: Cocatalyst-Free Open-Circuit Hydrogen Generation

Here we report that TiO2 nanotube (NT) arrays, converted by a high pressure H-2 treatment to anatase-like black titania, show a high open-circuit photocatalytic hydrogen production rate without the presence of a cocatalyst. Tubes converted to black titania using classic reduction treatments (e.g., atmospheric pressure H-2/Ar annealing) do not show this effect. The main difference caused by the high H-2 pressure annealing is the resulting room-temperature stable, isolated Ti3+ defect-structure created in the anatase nanotubes, as evident from electron spin resonance (ESR) investigations. This feature, absent for conventional reduction, seems thus to be responsible for activating intrinsic, cocatalytic centers that enable the observed high open-circuit hydrogen generation.